Here are a couple places to get them. I read else where there is a special too to clamp them in place and you torch them red hot and pound the end with a special concave anvil on an airchisel. There is a build thread on here where a guy used some on his truck. He cut and stretched the rear frame upwards and mad new bed mounts and it looked like a production frame. anyone know which build that is??
http://www.bigflatsrivet.com/install.html
http://www.grainger.com/Grainger/Sol...U12?Pid=search
(i dont know what size this is just a sample)