The 3.5" hard-drives use a 6-32 screw around a 1/4" long.

The 2.5" Hard-drives, CD-Roms, Floppy Drives, use an M3 screw around 6mm long.

And you can probably find the 6-32 screws at a hardware store. Might need to snip the ends to length. Good luck on the M3 if you need them. Most hardware stores don't carry much smaller then M6.