http://www.mcmaster.com
If you want to find regular hex head screws, type "screws" into the search box. Then click on "Socket Cap Screws". A list of many different screw types will appear. To choose the 12.9 screws, just click on "Class 12.9" under the Class heading. Unfortunately it looks like 12.9 is only offered for the standard head hex screws... And the countersunk screws can be gotten in 10.9 at best, which could explain why the hexes don't last as long for me compared to standard hex head screws.
Also, McMaster sells washers, hex nuts, lock nuts, etc. in bulk as well, and it really is a great deal to buy these pieces by the 100 if you are into custom building like I am.