The U.S. and Canada use 8 1/2 x 11 paper as standard. Europe uses A4, and I'm under the impression - based on no concrete evidence - that most of the rest of the world does as well. But 8 1/2 x 11 is standard here, so that's what all the shops sell. That, and legal sized paper.
I started off at Staples. The sales assistant assured me they stocked A4, but had to go find a manager to find out where. He came back disappointed. They didn't stock it after all, but I hadn't expected they would.
Grand and Toy was the obvious next stop, the Avenue and Bloor branch. As I walked over, I pondered what specialty stationary stores were nearby, in case they had it. The sales assistant at Grand and Toy was confused. I think she thought I'd said "84", meaning the weight of the paper. Once she realized she had no idea what I was on about, she sent me to the order desk at the back. I treasured vague hopes that some other one of their downtown stores might have stock.
But then the manager commented that she thought they had some in stock... and they did! I found A4 paper in Toronto! And the sales assistant gave me 5% off for having misguided me.
I don't know why a Canadian store stocked A4, but I needed it, they had it, and I went away a very happy customer.