Without a nice urban industrial machnists supply shop nearby, find a Lowes/HDepot and take the other screw with you. Their fastener section has a board with a bunch of male/female bits to match.
TIP- expect the have to cut the new bit to the correct shorter length, buy matching nuts in advance, thread them on an oiled screw first, then cut, then run the nuts down to clean the burr from the cut.
Same stores also sell 'contractor brooms' big magnets on a stick, great for cleaning up steel case ammo, and finding screws...