I've used Southern Brewing in Tampa and the Beer and WM Pantry in Pinellas Park. Both are good stores - the Pantry is geared more towards wine but has a sufficient selection of grains (about 20 or so), yeast, hops and equipment. The owner (Mary) and staff are very cool and knowledgable.
SB in Tampa is like a freaking candy store - about 80 grains and bulk if you want it. Extremely knowledgable staff and a full selection of pretty much whatever you need for supplies and equipment. They are a wholesaler to commercial brewers and have adapted to acommodate the homebrewer much better over the last year or so.