Are you talking about the kind where you just sit on it, it's backless, and you use the car's seat belt?
We just bring our own. You can get them for $15 at the store, so even if you just lug it one way and then leave it behind, it's probably cheaper than renting one for a week.
When DD was in a full size car seat I did rent one with the car once and it was fine, but othertimes I took my own seat because I just felt more comfortable with a seat I was used to (plus the fees to rent one are so high).