Do they sell anything to put under a carseat/booster seat that protects leather interior and helps not to let?

you could also use a towel that matches your interior a lot cheaper way to go.

Iv'e always used a towel and it has worked fine.

Yes, they do, and you can get them at Babies R Us. I'm sure you can find them all over the internet, though,

I have leather seats and what i did was used the lock clip and got help had someone sit on it for me and it made it so tight in my car that i didnt slide. you will need help though to do this its not easy!