Take one of the others to Denaults Hardware where they have the boxes with every size scriew in it. It will likely be metric. match the threads with the screws in the boxes. they also may have a thread gauge mounted in the area you can screw it into.
Once you know what thread it is, measure the length of the threads only in mm.
McMaster carr has a good selection of most screws and you can probably find what you need there.
Jerry