I bought my cup discharger rifle about ten years ago ($75 complete) from a company in Penn Run, PA, called Springfield Sporters. Just went to their web site and it seems to indicated they are parts only now. Might be worth a call if you are interested in either a whole unit or just the cup discharger.
http://ssporters.com/index.html
When used in a living history display, the rifle always elicits lots of questions and comments. I bought a resin Mills bomb and made it up with the plate on the bottom to go with the rifle.
Something I have always wondered is how much it was used after D-Day. From much of what I have seen, it seems the 2" mortar pretty well took its place by then.
Tom