I bumped someting right at the end corner of my Dually's rear fender. Is there a recommended way to repair a hole in this plastic body part? If not, does anyone sell the piece (the whole bedside, plastic fender) on-line?

Yes they are repairable - far cheaper than replacing IF you have the pieces AND the hole is small.

If you have the pieces for a relative small hole, this is a do it yourself job with some clear epoxy and a bit of care. You will need a mild solvent to wash off any excess epoxy from the finish, enamel reducer should work. If the pieces aren't available there are repair kits that may work providing the area isn't too large. Go to a body shop and get a professional opinion.