Hi! To get anything affordable, I'd recommend buying off Ebay. You can get some really cute stuff for about $10 US a piece with reasonable shipping rates.
If you don't mind spending a bit more, there are a few boutiques around town but they're pricey. One place that I plan on checking out once I get my little prince is Mona's at Dufferin and Queen. The website is
http://www.monasforpets.com/shop.cfm.
Oh! And I just remembered: there's a kiosk (just a little place in the middle of the hall) in the Eaton Centre downtown on the lower level near Shoppers. They have the CUTEST little outfits and accessories for dogs! You HAVE to check them out!