I've got an old Plano belly pack that I've been using for spin/baitcast fishing since '98. If I could find another like it I'd try it out for fly fishing. As it is, I have a vest for the fly gear, a store brand I got at Sportsmen's Warehouse for ~$30. I probably have a lot of stuff in it that I don't need, and don't have stuff in it that I do need, but I'll work that all out as I get more experience.

One tip for anyone shopping for a vest: if the one you're trying on has a "back pocket" that can double as a creel/lunch box, make sure you can reach it easily. I didn't realize until I was on the river that I couldn't reach my water bottle without either taking off my vest or having one of my fishing buddies grab it for me. That was a bit irritating.