Not trying to be a butt head, but unless you just like the looks of a thumbhole stock, why put one on it. I always wanted a thumbhole on mine, not for looks but for funtionality. Then I realized that the standard stock is basically a thumbhole with the part of the stock above the thumb removed which means you do not need to remove your thumb from the stock to cock it.
The thumbholes do look pretty cool. Hope you find one. Gunstocksinc sells them but I think theirs are either right handed or left handed. They will however make a custom stock I think, so you might want to give them a call. Their price would probably be reasonable