Little update:

I went to the Bay as suggested. The sales person was very helpful and when I asked if they sold spare parts, he explained to me that this is also a liability issue and wheels always have to be mounted by professionals as suitcases with failing wheels could become hazardous to other people on an escalator.
Unfortunately, none of the “bring it back to the manufacturer/place of purchase” worked for me as I hadn’t kept the receipt/warranty card.

Since I had to go to YTZ a few days later anyway to meet a friend, I used the opportunity and took my empty trolley. The lady at departures was a little reluctant at first “this is Air Canada Express, not Air Canada” and “you should have left your trolley at YYZ” (hmmm, and carry 25 pounds of contents home in plastic bags?), but then said she would advise the person at baggage claim of my issue and directed me to go to that area. When she arrived back from meeting an arriving claim, she already knew about it, and quickly assured me she would handle this. A bit of paperwork later, she gave me an updated print out for my claim file and advised she would send the trolley to YUL with the next machine. This could take 6-10 business days which I thought was very good.

Only four days later, I had a notice from FedEx that they had tried to deliver something but I wasn’t home. I called them to see what it was (didn’t expect it this early) and if they could just drop it off. That didn’t work because they required a signature. Fair enough. I picked it up from FedEx – the wheel was perfectly replaced and the trolley runs as smooth as new again.

After a bad start, this came to a very positive end. Very pleased with the Air Canada Service – and possibly a good option for others who don’t want to make the trip out to Mississauga/Thornhill.