BestThe best source is your local dealer. Depending on how good the parts diagrams are, it can be a pain ordering the correct bolt. If you do it online or over the phone, there's a good chance they'll order the wrong one. Go to the parts counter and look at the picture with the parts specialist.. hopefullyHopefully, the exact bolt is easy enough to point out.
Chances are, they'll have to special order it. If the wrong one comes in, you don't have to worry about shipping back and forth.
There are some really bad parts diagrams out there, even in official dealerships.