I looked for those when I had the same problem, but without success. I wound up just buying a new bridge and taking one of the height screws from it. So if it happens again, I have more. Not to mention saddles and intonation screws.
Over here in England they're called "grub screws" They seem to be readily available, just make sure you get the right thread size! As long as your bass isn't a scary 1960-1980 Japanese model, it should be standard