Go to the Lehigh Warehouse. They also have patchboxes, sights, brass castings. While you're in the neighborhood, stop in at the Christians Spring Gunshop and Supply and Ale House.
There may have been call once upon a time for batches of springs to be made. But the market today is so small, it's not worth tooling up for. Nate, I think you're stuck making your own, at least until this flintlock technology becomes popular.