I have two Aspen Canine Seat Belts for Indy and Ava. They are available at Petco and Petsmart. There are also others that are similar and may be as good.
A helpful hint: the instructions show the cars seatbelt passing throught the loop on the canine belt. The problem is that it really pins the dog down and doesn't allow him much movement to get comfortable.
I went to Home Depot and got two stainless steel links that look like the links in a chain. The have a threaded section on one of the sides that can be opened. I opened the link, slid it over the cars shoulder belt, not the lap portion of the belt, and through the loop on the dog's belt. MAKE SURE TO COUPLE THE VEHICLE'S BELT. It allows the dog to stand, sit, or lie down, and shift positions. It allows free vertical movement along the belt, but does not allow forward motion during hard stops.
There is a picture of Indy wearing his in our gallery.
Tom