Jesel sells an excellent quality bushing:
http://www.jeselonline.com/index.php?categoryid=82&s=ea153fcc7956f37adfd37898c7743868
So does Precision Performance Products:
http://www.pppcenter.com/06Catpgs/C018.pdf
I am sure that there are common commercial bushings that can be used. Before this process became commonplace, we used bushings from Sealed Power and Dana that were called king pin bushings. They just happened to be a size we could cut to fit and then ream to size after they were installed.