The Rare Mushroom doesn't take the place of one of your fossils, does it? I can only find three fossils in my town today and it's driving me bats.
It's been a while but iirc you need to to have one of those special tree stumps that spawn mushrooms. Special stumps are designs that don't resemble a bulls eye target.
again iirc, If you already have mushrooms around the town. Remove them and put them in storage so when that special mushroom appears it will be easier to obtain the mushroom series...
I'm too lazy to recheck this right now and my memory is getting hazy.
EDIT
Also if anyone needs to order stuff from Ganon he's in my RV