Cheapest Canadian site I've found is rittenhouse.ca.  There's also spwarehouse.ca, but they didn't seem that cheap for the few things I've compared.  They've got more big-farm stuff than Rittenhouse though. 

 Plastic-mart.com (US) seems to have great prices on tanks.  Tank and cradle prices were a titch under half (not incl. delivery) of what my semi-local (2hr drive away) farmstore wanted.  Plastic-mart has not returned my email for a price quote for shipping to Canada though, so obviously their customer service and people skills are unreliable and, apparently, they won't ship here.  Their policy seem to be "If it takes a few seconds and doesn't make us money, we can't be bothered".