One of the screws on my left foot cleat is lost. Do I have to buy a new set of cleats, are spare screws available, or should i just try some spare screws lying around that may work?

And what cleats are you using...?

the ones that come with Shimano PD-520 pedals

Depends on whether or not you want to get it out again. If I were you, I'd pull the other screw and go to the LBS. You want the hex head for when you take them out someday.

or a hardware store.

ed rader

Although better hardware stores will have something with the correct (M5) threading, the heads of SPD cleat screws are specially shaped to fit the cleats. You won't find them at any hardware store I know of, but any decent bike shop should be able to provide them.

Sheldon "M5" Brown

Really? They're not the standard metric 90 degree flat head? I honestly don't know but I'd be surprised if they designed in a special screw where it seems to me that a standard one would work (though I haven't looked that closely, just going from memory during installation).