I made one out of wood, basically a box fame out of 2x4's. Kinda ghetto, but needed to haul 2 kayaks to baja when I was young and broke. I just pulled it out of the bed when not in use, stored it in my backyard.
You can look for something like this below. $160 for the pair, of course, you'd have to go to Santucky.
http://www.bigwatersedge.com/bwevb/s...4&postcount=15