Not sure if you are asking about reef shopping or shopping in general...

For reef equip, etc., here is a site that lists a number of LFS's in the GTA

http://metricksystem.com/chris/lfs.html

If you are talking about general shopping, try:

Eaton's Centre (Yonge St. and Queen St. - downtown)

Square One (Hwy 403 and Hwy 10 - close to the airport)

Mark