Buckeye , check out some of the gun parts dealers on the web. Just last month I bought a 375 and a 356 barrel on ebay, both were unfired. They are out there , just not easy to find. Be ready for a little sticker shock. The people that have them will know how hard they are to find. HYDE
Any of the well known barrel makers could easily handle this. do a search on Shilen, E R Shaw, Douglas, etc. Also check www.e-gunparts.com They sometimes have original Marlin barrels in odd calibers for decent prices.