Depending on your rental agency, they may offer seats free of charge or for a small fee. I know AVIS mentions seats. I'd check with the rental company.
Sorry, I read your post again and it looks as if you won't be renting a car. You most likely won't be able to get a seat from a rental agency if you're not renting a car.
[This message has been edited by nevado (edited 03-08-2001).]