Car seat base question

I'm kind of lost on this. I bought a Graco travel system, brand new, and now I have to buy a second car seat base for the other car. Does anyone know if it have to be the exact same car seat base, I know it have to be the same brand, but Graco sells two different kinds of car seat bases. I don't really want to spend so much on just a base, so I've been looking on Craigslist and I keep seeing the one that mine didn't come with, so that's why I'm asking. I would really appreciate the information, thank you.