It is Perfectly legal in Canada to have an AR upper with a 14.5" barrel. Or 11.5. Or 10.5, or 9", or 7", etc..
The problem is the supply, and the US state department. The state department will NOT allow the export of upper recievers with barrels shorter than 16" without an end user certificate, which are only available to military/LE. Since most AR15 manufacturers are American, that basically screws all of us over. There is the option of Dlask, but...well....search their name on this site, and read the threads. I don't have any personal experience with Dlask AR parts, but I'm wary from what I've heard.
However, it is possible to get your hands on Short barrels. I'm not sure how, because I thought that the State department restrictions covered everything, but I guess not. I'm fairly certain that Wolverine Supply has them available, and I've heard that Questar does as well.
What you should NOT do, is have the barrel cut down. Most AR15 barrels are chrome lined. This is good for a shooter, because chrome lined barrels stand up much better than non-chrome lined barrels to the fast shooting, and high mileage that ARs are usually subjected too. Also, you don't have to worry about scratching a chrome lined bore with a steel cleaning rod, because the chrome is harder than the steel. The problem is that if you try to cut a chrome lined barrel, there's a very good chance of the chrome CHIPPING at the cut off point, and that, obviously, is not a good thing. If you took the barrel to a gunsmith,and asked them to turn it down, they would probably say no, for that exact reason.
Because of the problems with importing short barrels for AR15s, I was looking at trying to cut down several AR barrels as well, but the general consensus from gunsmiths & weapons techs that I talked to, was that I was better off buying a new (or used) barrel/upper.
SO...........I just wrote this to give you some perspective on WHY it's hard to get shorty uppers. Check with Questar, Wolverine, in fact, check ALL the AR15 dealers, and hopefully you find what you're after!
Good luck!
Tim