BMW sells a rebuild kit (ATE parts) or try one of the big BMW Aftermarket mailorder places like Bekkers. It includes new rubber seals, dust seals and the spring clips that hold them in. But, you would not normally separate the caliper halves to rebuild, just remove the pistons and seals. If pistons or bores are damaged, you'll have to replace the calipers or have someplace like Stainless Steel Brakes rebuild the caliper with new stainless bores and pistons (not a quick solution). OR... do as many have done and install E32 calipers... single piston, floating caliper(get them used or new). Very slightly shifts braking bias toward the front because of slightly larger piston area.