Patrick, here is a link to an online rivet manufacturer.
https://hansonrivet.wpengine.com/rivets ... nd-rivets/The PASD603H through PASD610H rivets are 3/16" diameter closed end structural aluminum rivets with a steel mandrel.
The length is determined by the grip range (i.e. the thickness of the materials that you are riveting together).
The grip range and the drill size are extremely important for a strong bond.
You could order on line, but you could probably find them at at your local industrial hardware supply.
Good luck with your build. Fred