We have one by Outward Hound. It was $20 at Petsmart. I believe it is meant for the front seat but we use it in the back seat because we have a split bench. You have to have either captains chairs or a split bench AND a headrest to put it in the back, but we do so that works out well. Loki can curl up in a ball and sleep or he can sit and look out the window. I bought him a step in harness that I can put right over his clothes or whatever to attach to the leash clip. I don't feel good about attaching it to his collar should we get in an accident. He has had his seat since we first got him so he knows he can't get out and he must sit quietly.
Petsmart
This hangs off of the seat. The picture shows it resting on the seat but that's not how we have ours set up.