I bought individuals from Mancini Racing. However, the ones they show now don't show the relief cut on the outside (as shown on their pushrod kits).
Mancini Racing Brake Pushrod Bushing
It looks like 70-72 have a rectangular hole and the wheel cylinder has a 3/8"-24 inlet thread. In 73 they still had the rectangular hole but the thread changed to 1/2"-20.
http://www.jimsautoparts.com/brake.htm
They offer them in a kit if you are desperate. I was able to find a reasonable match at my local parts store as well (Napa).
Jeff