Not exactly a pillow, but I have good luck using a bag of frozen veges on the lower back or alterantley a larger plastic bag type gel freezer pack advertised to be ised like in a lunch carrier. I dont keep that one fully frozen as it becomes to hard with sharper edges so either keep in fridge only or allow it time to partially defrost before the long drive. I haven't found the pillow supports to be helpful at all.

I bought a thermapedic pillow ( novaform) from a childrens store...it's small and pretty flat..none of those undulations as in the adult size pillows. I think it would work really well as a lumbar support. It would fit your back and has firm but giving support. I use mine for sleeping because of neck issues. I got it on line from a children's store.

Be careful when buying one of those. I bought one last Christmas as a gift and it was a horrible idea. The one I bought from The Sharper Image was too bulky and made driving uncomfortable. Twinkle is probably right with the pillow from the children's store.