The actual circuit boards are held on with speed nuts. There are studs from the gauge itself. Or do you mean the self tapping screws the hold the gauge panel to the bezel?
Speed nuts, guessing, 10/32? They are just a thin stamped nut, hardware store. The others , self tapping, are probably close to the same size and have a 1/4" head. Not going to find those at a hardware store, but could substitute something. Where are the originals?