Well, barrels are not necessarily prohibited from export to Canada but they do require an import license to be obtained. I suppose this could be enough hassle that it effectively acts as a ban.
What bore diameter do you need? In the U.S. pre-WW2 .45 Colt barrels are typically .454" diameter. Post-war barrels are usually .451". Looks to me like you ought to be able to find an old S&W Hand Ejector 2nd model barrel in .455 caliber somewhere in Canada, since so many were shipped there. They're 6.5 inches long and were produced using .45 Colt tooling so their bore diameter is actually .454".
Another plus is that they are slimmer than the barrels Colt made so there's less machining to do.
-Bob