The factory trays do pop up in the classifieds here from time to time. I think they came with some trailer packages and not just in the 6.2L trucks. If you don't want to pay $60 to LMC truck, you could get a universal tray or random junkyard tray and build your own support brackets. You might find a NOS 15616327 tray somewhere, but you'll be lucky if the price isn't real high.
Hasn't anybody tried knocking the bracket off a RH tray and putting it on backwards to make a LH tray?