Leachco also make one called prop-ease. it's really good because it's like a long sausage that you can adjust around the head and body. i got one for about $25 from toys'r'us. but have a look on the leachco website (just leachco.com) and click on travel. there's heaps of different ones
A WORD OF WARNING: I used one of the pale yellow ones from target in evie's carseat when she was a newborn and after a while she started SCREAMING, when i pulled over and went to her she was pouring with sweat. so just make sure there's enough ventilation and stuff so baby doesnt get too hot