Just a guess, but most use heavy wall tubing and then thread it to make a adjustable end.
go to places like thor bros, etc to see their kits, ask what you get in a kit so you can source material......
here is a link to a google search I did, talk to some of these folks.....
https://www.google.com/webhp?source=...uspension+kits
thor brothers:
http://*************/catalog/4-link-parts