I actually have two sets. One set are home made and bolt on the the frame of the truck with some square tubing I welded up to fit. The flaps themselves are actually one semi mudflap cut in half for each side. The other set only go on when towing and are made by Enkay. They fasten to the truck's regular trailer hitch, and with the trailer hitched to the truck, ride very close to the ground. Even with two sets, I still get some cast off on the trailer, but nothing like would without them.
The lowest price I found for the Enkay's was at Cabella's.
The Home made flaps:
The Enkay Flaps: