A shiny Blastoise always starts the same way: hunting a shiny Squirtle (or Wartortle) and evolving it. Squirtle has been available in some form in nearly every mainline generation, so here's every game where you can find one, what system each runs on, and how to actually hunt it - with extra depth on the Switch and Switch 2 games where it matters most.

Red, Blue, and Green (Game Boy). The original method: Squirtle is one of the three starters Professor Oak offers at the very beginning. Shiny Pokemon didn't exist visually in Generation I, so there's no shiny hunting here in the traditional sense - though see the Gold, Silver, and Crystal entry below for how a Gen I Pokemon's hidden stats could still make it turn out shiny once traded forward.

Yellow (Game Boy). Since Pikachu is the forced starter in Yellow, Squirtle instead comes as a gift from Officer Jenny in Vermilion City, given once you've defeated Lt. Surge and earned the Thunder Badge. Same as Red/Blue/Green, Generation I predates shinies entirely.

Gold, Silver, and Crystal (Game Boy Color). Squirtle isn't obtainable inside these games directly - the only route in is trading one over from a Generation I game via the Time Capsule. This is where Gen I's shiny mechanic gets strange: shininess wasn't displayed as a visual trait until Gen II introduced it, but the underlying DVs (the precursor to IVs) that Gen II's shiny formula checks were already being generated on every Pokemon back in Red, Blue, and Yellow. A Pokemon turns out shiny in Gen II if its Attack DV is 2, 3, 6, 7, 10, 11, 14, or 15, and its Defense, Speed, and Special DVs are all 10, odds of roughly 1-in-8192, identical to hunting directly in Gen II. So it was technically possible to already be carrying a shiny Pokemon while still playing Gen I, you just had no way to see it, target it, or confirm it. The only way to find out was to trade that Pokemon through the Time Capsule into Gold, Silver, or Crystal, where the game would finally render it in its shiny colors.

Ruby, Sapphire, and Emerald (Game Boy Advance). Same story as Gen II: no wild Squirtle or starter option in Hoenn. You'll need to trade one in from FireRed or LeafGreen.

FireRed and LeafGreen (Game Boy Advance). These Gen I remakes bring back the original starter choice, Squirtle included, complete with modern shiny odds (1-in-8192 in Gen III). It's a fully valid, if slow, shiny hunt via repeated soft resets.

Diamond, Pearl, and Platinum (Nintendo DS). Squirtle isn't native to Sinnoh. The only way in is migrating one from a Game Boy Advance game through Pal Park, which brings its shininess with it but doesn't let you hunt fresh ones in-game.

HeartGold and SoulSilver (Nintendo DS). After defeating Red at Mt. Silver, Professor Oak offers a choice of the three original Kanto starters, Squirtle among them. It's a standard gift Pokemon, so soft-resetting in front of the offer is a legitimate (if patience-testing) shiny hunt.

Black, White, Black 2, and White 2 (Nintendo DS). No wild or gift Squirtle in Unova. Getting one means transferring in from an earlier game via Poke Transfer or the old Dream World service.

X and Y (Nintendo 3DS). After reaching Professor Sycamore's lab as champion, he offers a choice of Kanto starter, and this one is notable: unlike most later gift Pokemon, this Squirtle is not shiny-locked, so soft-resetting in front of Sycamore is a genuine shot at a shiny.

Omega Ruby and Alpha Sapphire (Nintendo 3DS). No native Squirtle in this Hoenn remake either; trading in from X/Y is the only route.

Sun and Moon (Nintendo 3DS). Squirtle isn't in the Alola dex in the base games - trading is required.

Ultra Sun and Ultra Moon (Nintendo 3DS). These add a genuinely wild Squirtle, found via the Island Scan QR Code feature at Seaward Cave on Melemele Island, available on in-game Mondays. It's a real wild encounter you can reset for, not just a gift.

Let's Go, Pikachu! and Let's Go, Eevee! (Nintendo Switch). Squirtle is a wild spawn on Routes 24 and 25 north and east of Cerulean City, and around the Seafoam Islands east of Cinnabar Island, plus available as a guaranteed gift from a Police Officer in Vermilion City once you've caught 60 different Pokemon. The Catch Combo system applies here too: chaining consecutive catches of Venonat or Psyduck (both common in the same areas) helps lure Squirtle out, and raises the shiny rate in tiers, from the base 1-in-4096 under a combo of 10, up to 1-in-512 at a combo of 21-30, and better still at 31 and beyond.

Sword and Shield (Nintendo Switch). There's a Gigantamax-capable Squirtle gift from Dojo Master Mustard at the Isle of Armor after beating the first trial, but it's shiny-locked, so it won't help a shiny hunt directly. Standard breeding - Nursery, a foreign-language Ditto for the Masuda Method, and the Shiny Charm once the Pokedex is complete - remains the most reliable method here.

Brilliant Diamond and Shining Pearl (Nintendo Switch). Squirtle is a wild encounter in the Grand Underground, turning up in Fountainspring Cave, Riverbank Cave, and Still-Water Cavern, but only after obtaining the National Pokedex. Grand Underground spawns can be reset by leaving and re-entering a Secret Base area, making this a workable direct hunt.

Legends: Arceus (Nintendo Switch). Worth noting for completeness: Squirtle was left out of the Hisui Pokedex entirely, the first mainline game to omit the Kanto starters. There's nothing to hunt here.

Scarlet and Violet base game (Nintendo Switch). Squirtle doesn't spawn in the wild in base Paldea, so you'll need to bring one in via Pokemon HOME first, then breed it at a Picnic with a foreign Ditto for the Masuda Method. Eating an Egg Power sandwich speeds up egg generation, a Flame Body Pokemon like Talonflame or Volcarona in your party cuts hatch time, and completing the Pokedex unlocks the Shiny Charm for roughly a 1-in-512 combined rate. Blastoise itself also appears as a Tera Raid boss during events, but that Blastoise is shiny-locked - useful as a battle, not as a hunt.

Scarlet and Violet: The Indigo Disk DLC (Nintendo Switch). The most direct method on any current-gen game: Squirtle is a genuine wild spawn along the southern section of the great river in the Terarium's Canyon Biome, unlocked by completing the "Boost Biodiversity" Support Board request there. A Water-type Encounter Power sandwich raises the odds of running into Squirtle specifically, turning this into a proper overworld shiny hunt rather than a breeding project.

Legends Z-A (Nintendo Switch and Nintendo Switch 2). Squirtle becomes available in Wild Zone 20 after finishing the main story, spawning near the stream at the center of the map. It's notably the rarest of the three Kanto starters to find here, with a very low catch rate on top of that. Fast-traveling out of and back into the zone resets its spawns entirely, so reset-and-rescan is the core loop. Mable's postgame research reward gives a Shiny Charm at level 50, pushing the odds to roughly 1-in-1024.

Pokemon GO (iOS and Android). Shiny Squirtle has been catchable since the seventh-ever Community Day, back on July 8, 2018, with Community Day Classic revisits on July 9, 2023 (re-run July 23 after login issues) and again in 2024. Community Day shiny rates run around 1-in-25, far better than normal. Outside of Community Day events, shiny Squirtle can turn up from ordinary wild spawns and eggs at GO's flat rate of roughly 1-in-500, regardless of weather.

Pokemon Champions (Nintendo Switch and Nintendo Switch 2). There's no catching in Champions at all, so traditional shiny hunting doesn't apply here. Blastoise itself is part of the Champions roster, including its Mega Blastoise form, but Squirtle and Wartortle aren't - Champions' roster only includes fully-evolved Pokemon (Pikachu is the lone exception). A shiny can occasionally show up as a random pull when recruiting at the Roster Ranch, but transferring one in from Pokemon HOME is the far more reliable route to a shiny Blastoise specifically.

Pokemon HOME (multi-platform). Whatever game you're actually playing, HOME lets you move a shiny Squirtle or Blastoise caught anywhere else into it, including from Pokemon GO, which is often the fastest source thanks to its flat, weather-independent rate. If grinding sandwiches, combos, or soft resets doesn't appeal, hunting in whichever game has the best odds and migrating the result afterward is a completely valid strategy.

For Switch and Switch 2 owners specifically, the Indigo Disk's wild Squirtle is the most direct hunt available today, Let's Go's Catch Combo system is the closest thing to a direct wild hunt without a DLC purchase, and Legends Z-A's Shiny Charm plus Wild Zone resets make for the newest hunting loop (patience required, since Squirtle is the rarest of the three starters to spawn there). Everything older comes down to trading, migrating, or a well-timed soft reset.