Yes, you can certainly buy an unlocked phone to use with Rogers. I would check independent cell phone stores (i.e., not those affiliated with Rogers, Fido, etc.) as well as Ebay.
www.howardforums.com is also a great resource for everything related to cell phones. If you decide to go with an unlocked phone, make sure the phone is suitable for 850MHz. Lots of unlocked (including those formerly with Fido) only have the 900MHz band and this is not ideal for Rogers.