There's one for sale in the "Accessories" forum here, even as we speak -- it looks like a fancy laminate one, though.
http://thefiringline.com/forums/showthread.php?t=442538
There are probably others if you look -- as
jersey_emt notes, people seem to replace the wooden ones with some sort of synthetic stock pretty often. You could try posting a "wanted" thread in that forum, as well.