OOOHH Boy! Lot's to unpack with that question. Some US gun shops and auction houses will ship to Canada and then it's up to you to do the waltz with Canada Customs. That can go smoothly or be a horror story. You can have a US vendor ship to a broker like Borderview but it'll cost you. Though there's no tax or duty on antiques, you are on the hook for the hst. I'll give you a perfect example. I fell in love with a very reasonably priced S&W 1st. Mod. DA in .44 Russian. It was a private purchase from a guy in the states. He did everything correctly except he put Ave. instead of Blvd. That was it. Customs messed up and to make a long story short, it took 3 months to get my pistol (see picture). As a rough estimate, if you get a good pistol in the US by the time you do the currency conversion, shipping, and HST, you're about double what you paid in US funds. If you want one to flip it's pretty hard to get something at a price range that'll make you a few bucks. If you want one for your own pleasure (mine is set up for bush carry and wilderness defense), then the price is less of an issue. Rock Island Auctions has some really nice ones coming up but nothing this old pensioner can afford.
