I'm not going to encourage you to break the law, but...if there is a law against owning .50 BMG dummy rounds in Canada, it sounds pretty difficult to enforce.
But there is obviously a huge difference between the U.S. having a law against export of these rounds (which affects only U.S. citizens) vs. Canada having a law against possession. If the former is the only thing stopping you, then I don't think it would be illegal for you to find a gun show across the border and pick some up.
Totally unrelated, but I own two dummy .50 rounds. Got 'em for $1 at a gun show about 10 years ago. They were a neat little souvenir, but if there are legal issues to consider in Canada, I can tell you that they're nothing too special. Mine mostly just sit in a locked box with my pistol ammo and I don't take them out much.