Some cars came with fold down child seats built in, but other than that, no.
What car seats are too tall? For your child, you might want to look into switching to a booster seat that is just the bottom, that way you still have something to direct the lap belt correctly, but if the child is tall enough to wear the shoulder belt correctly, you dont need something with a back. If your child isnt quite tall enough, then you can get one with an adjustable height back.
I don't know what else to tell you, the rear seats and seatbelts should accommodate a small adult, leg room aside. If you are having trouble with car seats that are too tall, you are just gonna have to look into different car seats or booster seats.
Your county social services office can actually give you quite a bit of advice and help direct you to car seats that may work for you. You may also want to consider giving them a call. In some areas your sheriffs office may also have programs available to inspect car seats or give advice.