Herba Mystica in Pokémon Scarlet & Violet can be found primarily by participating in high-level Tera Raid battles after you have beaten the main game. Here are the key ways and locations to obtain Herba Mystica:

How to Find Herba Mystica

• Tera Raids (5-star and 6-star):
  Herba Mystica drops as a rare reward from defeating Pokémon in 5-star and 6-star Tera Raids. The drop rate is about 10% for 5-star raids and 13% for 6-star raids. Some Pokémon drop multiple types of Herba Mystica, so targeting those can improve your chances. Examples of raid Pokémon that drop Herba Mystica include Gengar, Blissey, Glalie, Drifblim, Amoonguss, Eelektross, Palafin, and Cetitan among others

• After Beating the Game:
  You cannot collect Herba Mystica until after completing the main storylines (Starfall Street, Victory Road, Path of Legends, and The Way Home). Once completed, you unlock the ability to farm Herba Mystica via raids and other methods

• The Teal Mask DLC Method:
  In the Teal Mask DLC, there is a free method to get Herba Mystica by collecting all tablecloths from certain stores in Paldea and completing a timed challenge at the Olive Roll. After gathering all tablecloths and giving them to an NPC at Kitakame Hall, you receive multiple Herba Mystica as a gift

Summary of Pokémon That Drop Herba Mystica in Raids

Type of Herba Mystica| Example Pokémon Dropping It in Raids
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Sweet| Raichu, Ditto, Delibird, Sableye, Weavile, Froslass, Talonflame, Dragapult, Pawmot
Bitter| Eevee, Houndoom, Gardevoir, Camerupt, Magnezone, Rotom, Zoroark, Hydreigon, Volcarona, Clawitzer, Oranguru, Appletun, Toxtricity, Polteageist, Pincurchin, Arboliva, Armarouge, Glimmora, Tatsugiri
Salty| Slowbro, Cloyster, Bronzong, Hippowdon, Avalugg, Greedent, Corviknight, Coalossal, Copperajah, Garganacl, Orthworm
Sour| Slowking, Altaria, Mismagius, Gothitelle, Florges, Dragalge, Goodra, Mimikyu, Hatterene, Tinkaton
Spicy| Arcanine, Scyther, Paldean Tauros, Gyarados, Dragonite, Scizor, Tyranitar, Breloom, Slaking, Hariyama, Salamence, Staraptor, Luxray, Honchkrow, Garchomp, Abomasnow, Gallade, Krookodile, Haxorus, Braviary, Mudsdale, Tsareena, Passimian, Flapple, Grimmsnarl, Falinks, Indeedee, Ceruledge, Mabosstiff, Brambleghast, Bombirdier, Revavroom, Annihilape, Kingambit, Baxcalibur

Additional Notes

• Herba Mystica is a special ingredient used primarily for making sandwiches that grant various buffs.
• They were originally found in Area Zero (Great Crater of Paldea) historically but are now mainly obtained through raids and DLC methods.
• The DLC offers additional ways to get Herba Mystica, including the free gift method involving tablecloths

In summary, to find Herba Mystica, focus on completing the main game first, then participate in 5-star and 6-star Tera Raids to farm the herbs, or use the DLC method for free gifts of Herba Mystica by collecting tablecloths and completing the Olive Roll challenge