If you have an ace hardware around, simply go to Ace and get what you need. They will most likely have it. As for the size...take one with you, and then you can compare. I would get a bolt that is 1/2" longer than stock. Don't forget to get lock washers as well, and use some blue lock tite. Also look in a chilton for proper torque specs. Not torqueing it right can lead to them backing out.
Also, while you're at it, get a new gasket, and new differential fluid and change out the both on your rear end. Might as well start fresh so you don't have to go taking this stuff apart again.