I think you have a couple of choices;
1. is to buy a piece of deluxe grade walnut and have a stock maker make the stock or perhaps easier to simply have a stock maker supply the wood to your taste and make the stock
2. If you have a stock to copy, buy the wood and find someone with a good 3 dimensional pantograph to copy it then finish the stock yourself
Keep in mind that a piece of really nice grained european walnut, sufficient for a stock, is probably going to run $200 - $300. You can probably buy a plank of US walnut, 2" rough cut x 8" x 8 - 10' from a wholesaler for about $11 - $12 per board foot or about say $120 - $140
cheers mooncoon