Most kits come with hardware... that being said, I keep spare hardware on hand just in case.
The screws I use from hardware stores(Lowes,home depot) are stainless machine screws. I use either a nylon insert locknut or well nut(expansion nut is what they're called at the hardware store). Depends on location if I use a washer and locknut or wellnut. For size I find the #10-32x1" is pretty versatile and works for most applications. Sometimes I use a longer or shorter screw depending on the thickness of the accessory being mounted.
Most important thing is making sure they're stainless. Use goop if going the washer/locknut method, do not if using wellnuts.