It's a bit of a ride, but there's a place in Stuart called Longnecks, it's a brew-your-own place with a nice beer bar that also sells homebrewing supplies. They have a pretty good assortment of basic brewing ingredients and equipment. They carry White Labs yeast, along with some dry varieties. SW corner of Indian and Willoughby Sts in the plaza, close to both I-95 and the Turnpike.