Hopefully, your hardware store is well stocked.  5/8 bolts are not the most common thing in the world.  I do not think that my HD or Lowe's carries them, if they do, they probably do not have the small shoulder length that you need.  The hardest part may be finding one with the right shoulder size but you can probably get away with one that is threaded the full length, since it is just a pivot and not really turning on it or anything.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.