There are a number of gun builders in Canada who currently import and build custom .50 BMG rifles. There is a certain gunsmith in Alberta who shall not be named here that is turning out some really nice examples.
If you're only interested in mil-surp .50BMG barrels, as opposed to sporting/tactical .50BMG, bbls, your search may be a little more difficult. Might check with Marstar, IIRC, they had some .50BMG bbls they were going to use to re-barrel those 14.5mm Soviet anti-tank guns they brought in some years ago. They might still have some laying around.