Registration and Posting Agreement
just a heads up you dont need to keep posting the same question. someone will get around to you and give you a good answer. the only thing i know of right off hand is a junkyard but youre asking for new so i cant help you.
Google GM backing plate 12 bolt. You may have to make some modifications to what you find to make them work.
12 bolt truck is not the same as 12 bolt car.https://www.ebay.com/itm/12-Bolt-Rear-Backing-Plates-Drum-Brake-for-73-87-2wd-Chevy-GMC-truck-/273175442755?_trksid=p2385738.m4383.l4275.c10