Simple bubble wrap works. It's a PITA taking the tape off and storing it while at the show, but it works and if a piece is sold, the bubble wrap can be used to protect the print for the client when transporting it home. For boxes, I use the large boxes that frames I've ordered come in.
That said, because the bubble wrap is a hassle in terms of untaping, I decided that I'd simply use cardboard divders between prints in the boxes. I don't hang anything that's not framed anymore so the cardboard won't be rubbing on the surface of the print. I'll carry a roll of bubble wrap and have it on hand for sales but it'll cut down a lot of packing/unpacking/repacking time.