(1) Does it have to be authentic (like for a Concours-level resto)?
(2) You are referring to the bolts that hold the bracket to the spindle, correct? (If not, I am at a loss as to what you are referencing. lol)
If the answer to #1 is no, then you should be able to match the thread size to a proper length *Grade 8* bolt from a auto parts/hardware store. Even if you had to buy a longer one and cut it down to length, it's what I would do if I needed one.
*Because it is brake related, make sure that you use Grade 8 for safety reasons. A Grade 5 might be more than adequate, but overkill with brake/suspension hardware is always a good decision.