your friendly local hardware shop
Half a mile away the generic hardware shop has quite a good assortment of nuts/bolts/screws/etc in both SAI and metric sizes, and in SS, steel, brass and plastic.
I think this is sort of a package thing - some stores have it all, and some have nearly nothing. Look around in your neighborhood, you may find the place.
Otherwise, most cities will have a shop or two that specialize in fasteners. Let your fingers do the walking, before you go far offsite to find simple, standard parts.