I can't remember where I read about this technique, but I have found it incredibly
useful for scratchbuilding AFV models, and it could also be useful for adding detail
to kits.
Water filter canisters contain a mixture of tiny styrene spheres (between 2mm and fractions
of a millimetre in size) and charcoal fragments. The spheres make excellent rivet heads for anything
from 1/35 to 1/76 scale!
Cut open a used-up filter canister with scissors or a razor saw, then tip the contents onto some paper towels and leave them to dry (for as long as possible, it needs to be totally dry otherwise the bits stick together), and put them
in a plastic box or jar to store. Tip some of the mixture into a dish, and pick out the plastic spheres with tweezers. You can attach them by applying a tiny drop of glue to the surface of the model, then placing the "rivet" on with tweezers. Since they seem to be made of a styrene-type plastic, they stick easily with ordinary plastic cement.