I have had many good experiences with Track of the Wolf (Minnesota) and with Pecatonica Long Rifle Supplies (Illinois) and will be ordering some stuff from Muzzle Loader Builders Supply (Idaho) soon. They have barrels of all sizes from Green Mtn, Colerain, Rice and others so you can get almost anything you want. You can order directly from their online catalogues.
Regarding serial numbers, when I first registered all my guns at the last minute in 2002, I made up serial numbers for my home-built rifles and these were accepted by the CFC. A couple years later when trying to register a "Kit Gun" I suggested this same idea because of course there aren't serial numbers on the parts, and they almost came though the phone! They assured me that I must get the new serial number from them. I can only guess that because I am not a licensed manufacturer, they want to make sure the new Ser No. is unique. Regardless, I expect that if you simply gave them a made-up serial number for a new home-built gun that they would not know any better and it would be accepted... Let us know if you try it and it works.
Finally, I asked if I could strike the CFC serial number on the barrel rather than use their sticker and they were fine with that... even on the under side, out of sight was okay with the CFC. Given that the long gun registry might disappear soon, I will use a sticker until such time as the registry is gone then peel it off and discard it, rather than permanently mark the barrel with a stamp.