Yes, we've hired car seats along with a rental car before - with two small kids and two car seats, we couldn't have managed all of our luggage. When we had just one child, we took our own seat from home.
The seats were not the same quality as ours at home, but met the legal requirements and we judged them safe enough for the few short days we used them. We specially chose a car hire company that had multiple kinds of seats available for babies/toddlers/kids. If in doubt, call the hire company and ask about the child restraints they offer.