I had to replace both master and slaves on a 1969 Imp in the past. The slaves were done about 2 years ago. At that time, there were no replacements available, but I did buy rebuilt ones from Spryte Improvement (
http://wallace-id.com/foster.html) and was very pleased with the job. They will want your old ones so they can rebuild them for someone else. They posted a few posts up from this one (Snowcat Pat).
I cannot remember where I got the master cylinders from (perhaps RemoteAccessService.com), but if Pat from Spryte Improvement has them, I would feel comfortable getting them from him too.
I remember we fussed around for years trying to ream out the master cylinder bore and use a rebuild kit from Napa, but we never got it right. I think if it is pitted badly enough, you need to put a sleeve in it like another poster above mentioned.
Good luck.