They turn up occasionally on Gun Broker but they're costly. If you have time and opportunity, local gun shops (especially old ones) will often have a box of shotgun barrels you can dig around in. I've found hard-to-find barrels that way, but the dealer generally knows what they have, so while you may find a barrel that way, it may not be a huge bargain (or it may be - who knows?). If you like visiting little local gun shops when you travel, you can get lucky.

 

I think there is a plain barrel on eBay right now. As pbansen said, they are pricy.

Good luck and all the best.