Catan expansions are pretty easy to find in a lot of places, but you've gotta know where to look. Physical stores in Montreal might have limited stock, especially for the newer versions. Try specialty board game shops first (like small neighborhood stores rather than big chains) - they'll often order on request if it's not on the shelf. Otherwise, big sites like Amazon.ca, 401games.ca, or stuff like DesLivresque will have a better selection, and you get home delivery no problem.
For older or rare expansions, it's true that some become actually impossible to find because of reprints. Catan has put out several versions over time, and Asmodée (the French publisher) doesn't reprint all of them regularly. Marketplaces like eBay or Facebook groups for Montreal board gamers can be useful for finding used copies at reasonable prices. If you're just looking to expand your game, check what version of the base game you have exactly (original, new edition, etc.) and hunt for compatible expansions, because not everything works together.
Good tip too: ask local shops directly if they can do a pre-order for new expansions. That way you know you don't have to go out of your way, and often there are discounts on advance orders. It's worth asking rather than going around in circles.