Man those things are so frustrating to buy. They are such a rip off new. That said, I bought them new because I never found any used. I would suggest checking some junkyards as lots of vehicles used them (especially trucks with camper shells)
Don't waste your money! They are super simple to make. All you need is 4 pieces of 1" angle steel or aluminum. I made some for my Westy in about 30 minutes including paint.