i do believe you can steal them from paras and parasect. its rare that you'll get a big mushroom though (you mostly get small mushrooms)
the best way to do this though is to get some parasects and parases in the safari zone. it is about 90% of chance of them having either two tiny mushrooms or a big mushroom. the reason I say 90% is because I never caught a paras or parasect without them have a mushroom or two
paras and parasect will sometimes be holding mushrooms ( big and little).
Paris sometimes have mushrooms as an item in the wild. either catch them and hope they have small mushrooms or use the move thief to steal them, if they have them that is. Paris are everywhere but there are alot of weak ones in mount moon
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a mushroom is just like any other mushroom you can find tiny ones and big ones
if you have found a tiny mushroom, or a big mushroom, you can use that to teach new moves to your Pokemon. the move tutor is either in Fuchsia city, or Lavender town. you will need a big mushroom, or 2 little mushrooms. you can get them by catching either paras, or parasect.
Well, you either catch it, or use a Pokemon that knows thief without having it holding an item.
on two island next to the game corner, you need two "tinymushrooms" or one "big mushrooms", which (the tiny mushroom) can be found on the second level of mt. moon from parases. You can also get Paras and Parasect at the Safari zone but chances are slim.
Celadon City the same place as the big department store