The most common store that will care a very limited and, in most cases a different lenght or head type metric bolts or nuts, is ACE Hardware. Most of them will have M8, M10. etc., "generic" bolts and nuts, but limited to standard lenghts (you will have to cut off the end of the bolt if you want it to match exactly to where you need it). The head will most likely be standard Hex type without the much needed flange. Flange-head type bolts are used by many different vehicle manufacturers, but are very hard to find in the correct size.

Different thread pitch, such as 1.00mm, 1.25mm, 1.50mm will also be very hard to find at a local hardware store. This is because these sizes are translated into "Fine" or "Coarse" instead of the real measurement, so it gets very confusing and inaccurate, and then what you are looking for will always be hard to find.

Other popular stores include Home Depot, Lowe's, Orchard Supply and Hardware, even Walmart and Target will have a very small (funny, really) selection of metric bolts and nuts (that will most likely be completely different from what you need).