I had the exact same issue with Quaife mid-way through my build and finally my installer, Dennis Olthoff, was able to get a used ZF. You might try contacting Hillbank Motorsports in Irvine, California, V's Performance (also in Southern California), and your own installer if you are using one. I know how frustrating it is when this happens, but be persistent and keep checking!
Yeah, I know. I originally ordered my car from them, but with the taller-geared Quaife box (which they special ordered for me), then Quaife ran out of boxes altogether! Long story short: Everything worked out, as I got a ZF with the ratios I wanted. I think the moral of the story is to get the box you want early in the build process and hold onto it.