I've found a successful way to locate the microshroom in case a few of you were having trouble. I didn't bother scouring the ninja world for it, I went to the Hokage's office in Konoha where people have said they had success finding it there.
Turns out there a mushroom that will spawn and despawn there, and I only recall seeing it while on this quest, so if you go up the stairs from the area where Neji, Tenten, and Rock Lee are to the area where the Hokage's office is you should notice it. If you don't, simple walk back down the stairs and back up. I found the Microshroom after about 5 attempts doing this.
Just walk up the stairs and load the Hokage's area, punch the shroom, wall back down the stairs and back up to try to load the shroom again. If it's not there, walk down the stairs and back up. Rinse and repeat.