Mike,
W&N sells a disc brake caliper conversion from solid to vented ( widens the exisiting caliper ). The kit has the bolts, plates in your photo, and it seemed pretty reasonably priced.
Oem shows the part number is still good. So maybe a dealer?
Dorman, the people that make all sorts of specialty parts sold in most auto parts stores also sells caliper bolts. Not a whole lot of information there, but there is a practice of oversize bolts. Perhaps a machine shop can make you up one from a larger bolt/ stud. Can't be that hard.. if you can find a sympathetic machinist.
That caliper shows up on a Bav too. So maybe there's someone out there parting..
My two cents for what it's worth...good luck though.