I use some seatback protectors that I bought at babys r us or target.. They are by graco ( i think) and consist of a piece of water resistant cordura with a big elastic band that goes over the seat and sits at the joint of the bottom and back of the seat. The top is held on by velcro that wraps around the headrest posts. Works great. My seats would be ruined without them. My daughter kicks/rests her feet on the seat every time shes in the car. It is only going to get worse as she is in the 95th percentile for height for her age. At 3.75 years she is as tall as my buddies 5 yr old boy.
This is a similar idea to what I have:
If you are looking local I would look at walmart, target, and the baby stores. They all seem to have at least one option.
I take mine off and wash them occasionally, then spray it with silicone waterproofer.