On the 79-81 there are three pieces you'll need. Two "L" brackets and the plate that mounts to them. Also, check to see if you have the molly nuts already inserted in the space between the grills. If not, you'll need those as well.
As far as where to get them, I'd check Ebay first. There always seem to be two or three sets floating around at any given time.
FYI, if you don't have the molly nuts let me know. I have a box them in storage.
David