You could possibly get one directly from DISH they did make battery covers for older remotes but a dealer had to order so many it was cost prohibitive for a dealer to buy them. You might find someone on the forums that has a bad remote who will sell you the battery cover.
I called dish and they said it was no problem sending me a replacement cover. I don't have any receivers under warranty and don't sub to the warranty plan. Guess they see it as keeping a customer happy for the few cents a battery cover costs them.