In the Ferry Building is Far West Fungi, a small store that sells various types of fresh and dried mushrooms (lion's mane, morel, chantrelle, bluefoot, tree oyster, porcini, and plain 'ol button mushrooms, to name a few) books about mushrooms, truffle oils and truffle salts, and other mushroom-related paraphernalia, including mushroom growing kits. May is Garden Month on The Kitchn; imagine being able to harvest your own fresh mushrooms right outside your kitchen!
In addition to their location in the Ferry Building (which is open seven days a week) they also have stalls at various farmer's markets around the Bay Area. The staff is knowledgeable about all things fungi - take a look at their website, which contains a wild mushroom calendar, recipes, nutritional information, and a glossary of mushroom species. If it's about mushrooms - they have it, or they can find it.
Not in San Francisco? Don't fret; they have an online store.
(Image: Far West Fungi)