There was one for ~$130 I recall on ebay that several here bought and posted about. It appears to use a Chevy booster w/ an adapter for their threaded rod. They had to grind a rod shorter to not hit the firewall, but otherwise worked fine.
If you don't care about originality, a booster/MC from a newer car can work. I have one from a ~95 Breeze on my 65 Dart, using 73 Dart booster brackets and lever. That gives a light aluminum MC and no ugly rust. Was cheap at the junkyard and parts are easy.
