I have one called the Pet Stow-away. We just use the top part, not the suitcase underneath. I got mine for $55 + shipping, and I've seen them for about $50 at Pet Supplies Plus. It goes in any seat in the car because it attaches with the belt, but we put it in the middle of the back seat. We love it!
http://www.petsonthego.com/pyp/pages/pyp_globstow.html