Hi Farren. Interesting request. Didn't know there was anyone else interested in "shooting" these guns, other than myself. :haha:
There are probably three ways you can approach this:
1) You can have a custom builder actually copy the gun of your choice using specs and photos (many) of originals. This would be the most time consuming and expensive route.
2) Purchase an original gun you like that is complete, but in poor to fair condition, to keep the price low. The builder now has an exact original in his hands that he can duplicated. THEN, after the new gun is finished, re-sell the original to recapture some of your cost.
3) Another variation of #2 above would be to have a collector LOAN your builder a gun to duplicate. This is not really as hard as it sounds.
4) The way I do it - as Alden mentions above - is to buy an original in decent condition. Better than a wall hanger, but not so good to have a real collector value. Then refurbish it to firing condition. There are talented gunsmiths that can fix just about anything. At least that's been my experience. As well, original iron barrels can bored out and new STEEL liners installed for safe shooting. And, going this route is also the least expensive.
If your interested, I can send you some pics of these Ottoman Empire type guns that I had restored to firing order.
Good luck with the project. Rick.