I used to use the octagonal sprue from Airfix kits. I would shave off one set of opposing corners, and then whittle the other four sides down to match a nice hex shape. (It just takes a couple of minutes, either with a hobby knife, or a sanding block) Then I would stretch it over a flame to the desired thickness,and slice it up. One big chunk of sprue with about five minutes of work could give me a few hundred boltheads in varying sizes.
Also check out the automotive section of your hobby shop. S&S (Out of business) and Detail Master both made sets with nuts and bolts in photo etch. I would also try Replicas and Miniatures of Maryland. They had some too, plus they had a neat photoetch product called "Wizard Os' They were photoetched rings all stacked inside of each other. A zillion sizes per sheet. I used them for piston rings in a diorama of a guy rebuilding a car engine. They work well as gaskets, O rings, and should have some uses for armor too.
One last trick I picked up years ago at a model car show was to use straight pins. Just file the heads to a hex shape and cut them to length. I drove SWMBO nuts for a couple of years by putting the calipers on every straight pin in the house. Look for insect pins too. People who collect insects use very fine pins to ount them,and these make great smaller bolts. (In 1/25, they work well for linkages)
Good Luck,
Dave