I asked this question a couple of years ago, and didn't get an answer. The only source I know of is the dealer and they are expensive. There are actually 2 separate parts: The main backing plate and a smaller ring that I assume is to protect the bearing.

I have driven Saab 900s and Volvo 240s with rusted out front backing plates without any problems. The rest of those cars were rusty enough that I didn't think it was worth putting new plates on. Its a problem on the rear wheels since the emergency brake shoes slide on the backing plate.