any hardware store with hardware bins should have what you need. I useually find good replacment stuff from my local ACE hardware. Probuild has a good selection, but try ACE first. a lot of the big box sstores dont have the tiny stuff we need for RC. if there is a Fastnel dealer in your area then that would be best.
I also would sugest picking up a nice magnetic tray to put in your field kit to help avoid this type of problem in the future. Not a sure fire way to stop it, but a good magnet can help find lost screws in brush.