+1 to what mykeal said . . . .
you might also give Track of the Wolf a call . . . .
Also, keep track on eBay - you never know what will show up.
I'm assuming that you have all of the parts but the stock? Lock, triggers, furniture, etc.? If all you have is the barrel & lock - consider getting a stock preinlet for the barrel size and do your build up from there. I don';t know how much equipment and/or experience you have in regards to inletting, etc. but you also could always get a blank and make your own. If you go that route - price out a stock blank to see what it runs and then see if you can;'t locate the wood for one locally - you're probably going to need 8/4 thickness at least (2") or possibly 10/4. You'd have to measure your complete GPR at the lock portion of the stock and at the thickest portion of your buttstock - if I remember correctly the GPR has a cheekpiece? Whichever is the thickest portion will determine the thickness of the bland you'd need.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63