I went to a local speciality commercial hardware supplier that caters to construction contractors. Fastenal (
http://www.fastenal.com/ ) is where I bought them however any decent supplier should be able to help you out.
The bolts are allen cap screw (hex heads need ordered from overseas). Are 10MM x 80 MM long with a 1.25 fine thread. I use 2 washers on top and 2 on bottom and crank them down. Let sit overnight and check tightness again. Any play will be gone.