The nuts you guys are calling U-bolts are actually called "J" nuts. They slide over the edge of a piece of metal with an unthreaded holes and snap into place. Once in place, the "J"-nut has a threaded hole that allows one to use regular bolts.

The other option is to just use regular bolts/machine screws with regular nuts. The "J"-nuts are nice because you don't have to try and get your hand behind the panel to place a nut on the bolt.

Lowes has a great selection of automotive bolt, body bolts, "J"-nuts etc. in their pull out drawer section. I've bought a lot of them from there over the years...