This may sounds silly, but the screw shop. When I was fixing up my old scooter (at '71 Lambretta) I needed a bunch of odd things so called a few fasterner stores. They usually sell to tradesmen, machine shops and the like. If you're lucky you find one who'll help out finding the right size screws (and tell you if threads are stripped - if this is the case you'll have a harder time but there are things you can do!).