You can buy it anywhere and bridge it with a BlackBerry using any supported carrier. They're not locked to a carrier or using uncompatible bands like phones.
Unlike ipad, you don't need a data plan for your Playbook! You can tether and bridge to your blackberry phone and use your phone's plan for no extra charge. Now if you use more data than your plan allows....that will cost you. I have been with Telus for years and have had my Playbook since launch, so I know for a fact that Telus does not charge extra to bridge and tether to my BB