According to Consumer Reports Travel Letter, a number of the major agencies won’t unconditionally guarantee child seat availability.  We checked in on the latest: 

Alamo ( 800/GO-ALAMO) will guarantees a carseat with your rentals in the US and Canada, but they are subject to availability outside the US.

At most Avis Rent-A-Car franchises ( 800/331-1212), 24 hours' notice is required for a carseat reservation, at $9/Day or $45/Week.

Budget Rent-A-Car ( 800/527-0700) guarantees a seat if requested 48 hours in advance.

Dollar Rent-a-Car ( 800/800-3665) recommends reserving ahead of time to increase your chances of getting a seat.

Hertz ( 888/777-6096) requires that you reserve on line at the location where you will be picking up the car.

National Car Rental ( 800/CAR-RENT)guarantees child seats and requires that children under the age of 5 years ride in a federally approved seat.

Enterprise Rent-A-Car ( 800/261-7331)does not guarantee a carseat and you cannot reserve on line, but you must contact the specific location to make a request.

Thrifty Car Rental ( 800/847-4389) will not guarantee a seat, and you must request it in advance.

Bottom line: when making rental car plans, inquire specifically about child seat availability and get a guarantee with your confirmation number.