Check out Foxridge.com, or GreenMountain's web site.
The White Mountain has a 15/16 barrel to round and I believe any of Thompsons 15/16 octagon barrels will fit. the WMC is a 21" barrel, and this could be a prob, if you just want the carbine barrel. I have a WMC 50 cal. and have been thinking about getting another longer barrel for mine. Say like a 26" Renegade type.