It's important for the center speaker and the front-right and front-left speaker be 'matched'; that is, the same model from the same manufacturer. I would purchase a new three speaker set (center, right, left) and use your two current speakers and your two rear speakers.
I would encourage you to check out Fluance. They are mail-order only, from Canada.
www.fluance.com. That's what I'm using and I'm very happy.