If you're talking mail-order for computers and parts, I'd recommend these two. I have used both and been generally satisfied.
http://www.pccanada.com/default.asp (Etobicoke)
http://www.ncix.com/canada/index.php (Vancouver)
The first one has a list of links to their competition, so there are a bunch of other sites you can look up. Here are two more which look interesting but I have not bought from:
http://www.sprintcomputer.com/ (Ottawa)
http://www.accelerated.ca/acatalog/index.html (Winterpeg)
Mainstream stores like Future shop and Business Depot also list lots of computers and parts. Parts are generally of limited selection and more expensive at either. Canada.com has a pretty good listing of merchants under their "shopping" sub-heading:
http://www.canada.com/shopping/resul...211&cls_id=347
That should get you started.