Colts are really the only hard things to get in Canada because as was pointed out, Colt is precluded, by contract, from selling into the civilian market. Since they almost have a monopoly on LE and DND contracts, why care about the civilian market in a heavily gun-controlled country where sales will be limited to a small pool of black rifle enthusiasts? Pure economics. This also leaves the civilian market to other smaller players to try and make a go of it.
In that regard I've seen plenty of ARs for sale of all different kinds online and in stores. Its not like shopping for socks, but lets face it, if you're really into it, you're going to source out parts and custom dress it anyways. If you get yourself a decent milspec receiver you can build whatever you want from there. But buying exactly what you want off the shelf, unless its just a vanilla AR, you're going to be doing some research and parts ordering anyways.
I wonder what proportion black rifle enthusiasts/owners represent in the overall firearms community.