I'm not sure from your description but did your front brake come with the newer short mounting bolt but was missing the recessed nut or was the brake fitted with the old design long mounting bolt that uses an external nut?

If it has the current short bolt, recessed nuts available in a variety of lengths and, as noted, most LBS's have a bunch of extras.

If it has the old, long mounting bolt, either return it to the seller or your fork crown must be narrow enough to let it stick completely through so the external nut will fit. If your fork will work, you can drill out a short recessed nut with a 15/64" drill bit and use it as a bushing in the rear hole.