They almost always have them on-hand. The condition of them can vary, but if you get one that is worn/dirty, you can certainly ask for a replacement. I've also heard of one rental brand borrowing from another from the same parent company (Enterprise/National/Alamo, Avis/Budget, Hertz/Dollar/Thrifty, etc) if supplies run short.

Also keep in mind that when booking with Hertz you can get one free child seat if you are a AAA member and book with the AAA discount code.