Ingredients are guaranteed from missions. We get a chance at extra ingredients in some of them, in battles along the way in the dungeon.
Crystalmouths appear frequently when you do that mission, their catch rate keeps them balanced so you don’t get loads. Cryptamids you get guaranteed when they appear but they’re rarer.
All rare ingredients come up frequently enough in online missions, with quite a few being available every 16 days in that rare ingredients mission or freely available at any time (the two mentioned above).
I’ve never had an issue farming them or stocking up. If someone spends when they start the game and get lots of monsters quickly then they’ll take time to get enough ingredients to evolve them all. They can do that regularly enough I think it’s balanced as it is.