Take a Pokemon that knows Thief, Covet, or any other move that can steal an opponents hold
item. Go into Mt. Moon, down onto one of the narrow basement paths (if you're in the right
place, the only wild pokemon you'll find are Paras).

Paras holds a Tinymushroom half the time, and a Bigmushroom 5% of the time. Keep using
Thief on the Paras, remember to remove the mushroom every time you steal one, and
eventually you'll have a nice big stockpile of mushrooms.

Go to mt.moon. Paras can be found in those little p***age ways. Catch as many as you can.
They might have a tiny mushroom on them. To get a big mushroom you'll have to catch
Parasects in Ceurulen cave(spelling).