It is very hard to longer as most of the delivery services here will not handle live fish. Carriers like Canada Post, Purolater, Can Par and such will not touch your package if they know there is live fish in it. Makes shipping very expensive. Depending on where you are located there is http://www.belowwater.com/index.html but there is a high minimum. Where are you located? Hooking up with a local fish club would be a good bet. Try here for Canadian Clubs: http://www.caoac.on.ca/ .
Big Al's Online. When you go the the site for the first time, you have to choose if your in U.S. or Canada, and based on what you choose, the prices are different for currency.
If you are loking for salt water fish there is J&L Aquatics at www.jlaquatics.com or Aquarium Pros at www.aquariumpros.ca. For fresh water the only one that I know of is Below Water. Although they mostly have wild caught fish.