mushrooms?

Capture Parises to get small mushrooms and Parasects to get big mushooms. It should be attached to them and you can take them away.

There are a few ways to get the mushrooms. The easiest for me is to just teach a Pokemon thief and use it on a paras or parasect(once stolen take the mushroom away from the Pokemon you used thief with so you can thieve more).Oh, there is also another move that kadabra learns although I forgot what it is.

In the Pokétower (Lavender) you can find a big mushroom. I used the itemfinder to get it, from the top of my head it's the level above the one with the "sacred space" that works as a Pokécentre.

If you have already used the TM thief then their are other ways. Go to Mnt Moon and go into one of the passages with only which are rectangles or L shaped then look for paras and catch ever one you find. Then go to the box and remove the tiny mushrooms they have. This is a good way to do it but it's very time comsuming because not all paras have the Item.
Hope I helped

You should look for the mushrooms near the starting of the game it is more likely you will find one there.

You need to catch a paras it will most probably be holding if. Other wise use thief on a paras.