Generation 4:
- Platinum: Bought from a Move Tutor on route 212 for 2 Red Shards and 6 Blue Shards
- Diamond & Pearl: Not available
- Bought from a Move Tutor in the top left of Fronteir Access for 64 BP
Generation 5:
- Black & White: not available
- Black 2 & White 2: bought from a Move Tutor in Driftveil City for 10 Red Shards
Generation 6:
- X&Y: not available
- Omega Ruby & Alpha Sappire: bought from Move Tutor #4 (the one on the very left) at the west of the Battle Resort for 8 BP
Generation 7:
- Sun & Moon: not available
- Ultra Sun & Ultra Moon: bought from the Move Tutor on Ula'ula Beach at Ula'ula Island for 8 BP
- Let's Go Pikachu & Let's Go Eevee: not available
Generation 8:
- Sword & Shield: obtained by giving it TM04, which can be found in Wyndon City.
- Brilliant Diamond & Shining Pearl: not available
- Legends: Arceus: bought from Third Star Move Tutors for
2,000
Generation 9:
- Scarlet & Violet: obtained through TM69, which is obtained from the TM Machine for 5,000 LP, 3 Cubchoo Fur, and 3 Meditite Sweat
- Legends: Z-A: not available
Toxicroak did not exist in Generations prior to Generation 4.
Source: Toxicroak's moveset page on the PokemonDB
Hope I helped!
