You can get a sim card from Petro Canada for their phone plan. However, I have no experience with their service. I am also not sure how many Petro Can's you'll see out East. From what I've seen, the maritimes are predominately Irving gas stations.
PC mobile from President's Choice (I believe that would be Atlantic Superstore out east) have relatively cheap phones and plans, just don't know how cost effective they are considering that you only need it for 3 weeks. It uses the Bell network so you'll have more than just a local coverage area.
