It's probably best to get complete loaded backing plates. The shoes may look good, but the epoxy that holds them to the metal tables is prone to let goas they age or get overused in one manner or another.
As for the seals, you can go to any trailer repair shop or RV dealer, and tell them you need grease seals for 12" brakes. They will be able to sell you the seals as well as the backing plates.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.