Dorman Auto Products is your friend. This is the company that supplies most of the loose stock you see in the steel bins behind the counter. The web site is a little bit clunky, but with a little searching you can find what you want and then use the site to figure out what auto parts stores in your area stock the stuff. For example, my Spec Miata uses the lug nuts from a late model Ford Fusion (open ended, 19mm, steel, dirt cheap). I buy a dozen or so at a clip.