This is a classic do-it-yourself project. Too costly to have it completely stocked by a pro, not likely to find a factory stock. The big problem is finding a pattern. But, possibly you could convince a stockmaker, that makes stocks the traditional way, to head up a blank for a relatively reasonable sum and leave the rest to you.
I could never get Wenig to give any pricing on the phone. On the several occasions I called about different guns, they wanted me to send the gun to them for a quote. But, they may have a pattern. Can't hurt to call.
For an inexpensive gun like this, I'd try one of the cheaper stockmakers like Great American Gunstocks
http://www.gunstocks.com/ or
http://www.gunstocksinc.com/ But again, a pattern will be the issue.
A pattern could be made by taking an existing stock from any boxlock sxs and epoxy bedding the inletting of this receiver into it. Care would have to be taken to ensure proper drop/cast/etc..