I’ll throw in my 2 cents worth. There are others that know more about the big stuff than I, but I think you are somewhat limited by the 6" max height. There are still a few options, though. A couple things that come to mind are 1" Gatling and 37mm projectiles (without the case). Both of those should fit into a 2" x 6" space. Another option is 40mm Bofors projectiles, but there won’t be as much variation as the 37mm.
There are quite a few 37mm variants from several different countries from the late 1800s to WW2 (enough to warrant a 3 volume set of books on the subject “A Gun for All Nations: The 37mm Gun and Ammuntion” by Robert Mellichamp).
Not sure about shipping to Canada, although in theory inert stuff should not be a problem. But I’ve also heard stories about customs confiscating them, even though they are clearly marked “inert.” Maybe try gunbroker.com and look for the category Ammunition>Large Bore, Inert & Cannon Ammo.