The Civia Bryant should be available as a frame set, and has gotten some good reviews. Civia is distributed through QBP, so most bike shops should be able to order it for you (or online at a place such as universal cycles)

Spot Brand bicycles is belt-drive centric, but I'm not sure if you can get a frame only.

Many pre-existing chromoly frames can be re-worked by a frame builder to accept belt drive by cutting the drive side chain stay and inserting a Paragon stay splitter.

Other companies are producing bikes with the Gates system, but they are typically only available as pre-built bikes.