I have been looking everywhere for this with no luck. My Memotech Memopak RS232 Interface was stolen along with my entire ZX81 collection. Probably ended up in a dumpster somewhere Trying to rebuild a new system.
It seems they are quite rare. I've never seen one on eBay the last 10 years I've collected Sinclair. I don't know if anyone on this forum has one. If they have, they would cherish it, and not part with it.
I understand the Memotech Serial RS232 IF Memopak is rare and valuable. Furthermore I accept that if someone had one they would probably not want to part with it. That being said I would pay top dollar for one $500, $600 $700 would not be too much. This is the missing piece and linchpin of my system. I am hoping someday I will find one.