Also, there is BestBuy, Compusmart and Office Depot. But like the ones Howard mentioned, they too are very limited.
If you are looking for Dell related accessories, you'll have to go through Dell, ebay or other Internet merchants. However, if you're looking for more generic accessories, such as wireless routers, wireless CF cards, BT adapters, try places like
Canada Computers,
Logic Computer House,
Sunny Computers or
PC Village. I believe all of these stores are located in the Richmond Hill/Markham area, but there are also stores like this around the College/Spadina-Bathurst area I think. Note that for most of these stores, their prices are cash-discounted. Return/exchange policies are also not very generous. If you know exactly what you want, then these are the places to go. Don't expect any service/advice there.