Where does one go to buy any potential part for a Mazda (2006 Mazda6 I, specifically)?

The tire store lost the bolt that secures the spare tire, have been searching auto parts store on the web and can't find it. It happened months ago but I just realized today why my trunk has been making so much noise over speed bumps.

 

Any hardware store that carries Metric fasteners should be able to hook you up.