Petcetera and PetSmart are probably your best bet. Unfortunately, most of the doggie clothing stores based out of canada seem to be 'boutiques' and are quite pricey (especially here in toronto!). Not sure where in Ontario you are located, but Rens Pets Depot in Oakville is good for pet supplies — not sure what they have for clothing though.
Ren's Pets Depot Online Store
Buddy belts are my favorite harness - both my dogs have them. They are pricey off the get go, but I'm convinced you will NEVER need another harness again. They are made of leather and come in lots of different colors now.
Welcome to BUDDY BELT (A Division of Class Art Productions Inc.) if you want to check it out.
Congrats on the new little one!