Mushrooms seem to sprout anywhere a seed might spawn in the world. If you're playing Moonlight Falls, there are also mushrooms pre-planted on many residential and community lots (for instance, check the "treehouse" lot where the mischievous fairy family lives, or the house with the adoptive mother and her changeling fairy child).
At night, the "spotlight" mushroom is pretty easy to see from the map view.
Mushrooms can also be purchased at the new elixir consignment shop (and possibly the grocery??).
Some of the mushrooms (the porcini and white specifically) can be purchased at the grocery store. There are occasionally other types at the Alchemy store and over by the Gypsy Caravan in Moonlight Falls, there are several types of mushrooms growing. Also, at the Alchemy store in the back "yard" there are several types of mushrooms back there. I've also found a few types at Fae Ray's Gardens, where the Arboretum (sp?) is located. You can also find Life Fruit there.
I am an Angel who has tamed the Dragon. For I am NOT crunchy, NOR good with ketchup!