You can buy Mycelium at /warp grass for 5 grass. Once you place it, you can put either a red or brown mushroom on it and either wait for it to grow or bonemeal it. Once its fully grown, you just use a silktouch pickaxe to collect all of the blocks and thats about it!
Thats very strange. I tried it right before I posted just to make sure I wasnt giving any invalid information Is there enough room for the mushroom to grow? They grow rather tall sometimes. Is there also enough light? That could be a reason it might not be growing.
Yes, the big mushrooms need light. I would suggest buying 9 pieces of mycelium. Place them like this: ### ### ### Plant the mushroom in the middle one. Give it plenty of light and space. When it grows and you silk touch the mushroom, the middle block will become dirt. Since the dirt block is surrounded by mycelium, it will change back very soon.