For big ones I have used the decorative glass marbles that are flattened on one side, the kind you use in vases for table center pieces. They would be too big for most props, but look good as set dressing.
I've used "button caps", I think they're called. They're actually smooth, domed, and metallic, made to cover buttons so you can't see the button's thread holes I guess. They're hollow, like bowls, so you'll have to fill them (in my case I filled with hot glue for a 2-fer of filling and adhesion).
Try the small metal pins (brads they call them sometimes) used for scrapbooking and other craft, they come in all sizes and usually in bulk packs on ebay etc. Pretty cheap too. Just push or glue them in and paint, they would look the part.