Napa has them for about $25 bucks. They don't carry the secondary diaphragm or any of the odd parts like the secondary check ball. You can get those parts from John Earnyardt at Pony Carbs in New Mexico.
Last I checked Ford still carries the secondary diaphram. There is also a seller on fleebay that regularly offers a complete kit that includes the secondary. Looks to be of good quality. I'd have to dig for his name...