Recently we took our dog Charlie to buy a new harness and found K9 Couture at Yonge and Wellesley to be a dog owners' heaven.
K9 has a huge selection of reasonably-priced clothing for your favorite pooch no matter how large or small. The offerings range from the ridiculous (tuxedos, jeans, tutus) to the functional (plain/no-nonsense sweaters and booties to protect paw pads from salt and ice in the winter). If dressing up the dog is not your thing, there's a decent selection of leashes, collars, toys, treats, and grooming supplies. There is also a little bit in there for cats.
The layout of the store is a bit crowded so you might have to do a little digging to find what you want. Compared to some of the more frou-frou pet accessory shops out there, the prices aren't as high as you may think. This means that even if your four-legged friend hates their new outfit, at least you didn't spend a huge amount of money.