You can make your own with plastruct hex rod. Just cut the rod into slices of needed thickness. It takes a few to get ones with parallel sides, but you can get a lot of bolts from one package of rod.
Model Car Garage here, but I don't like PE for larger scale bolts, they are too flat.
Scale Hardeware also has small metal bolts, but they aren't really that cheap.
Check model railroad shops or sites, Grandt (sp?) and some other compaines make sprues of plastic bolt heads in different sizes and styles.
As for size, figure what size bolt you are replicating and scale it down here.