28.6mm clamp derailleurs are becoming less and less common anyway. I don't think SRAM makes any. It's very common for derailleurs to come with 34.9mm clamp and a couple of shims to adapt to 28.6 and 31.8 seat tubes. a little ugly but it works fine.
You can use a slice of 1" schedule 40 PVC pipe for the shim, should be about the right thickness.