If I may suggest...
McMaster-Carr is one of the biggest industrial supply outfits. They offer a huge catalog and have on-line ordering. (M-C is where i got the bolts you are asking about, but there are other sources as well.)
FASTENAL is another industrial supply store. They're smaller than MC, but do carry most everything for my cars I ever needed, including many electrical connectors (e.g., Weather Pack, etc.)
Check for local listings.
And, believe it or not, good ol'
ACE HARDWARE too has a decent selection of fastener type hardware including "grade 8" stuff like flywheel bolts, I have found.
Home Depot and
Menards too carry a good bit of nuts/bolts/screws/etc.
Anywayz... These are some of my favorite sources for fastener hardware of all kinds.