If I were you I’d consider joining the UMM club – join with Graham Potter the chairman – firstname.lastname@example.org
I think it’s a tenner a year and if you make it known to him that you’re after one I’m sure he’ll let you know what he hears of. Also an absolute fountain of knowledge of all things UMM and has quite a collection… maybe you could persuade him to part with one?
Otherwise I’d keep checking ebay, trovit, farm trader, auto trader and the other classifieds. Keep an eye out on here – I always post whatever I have seen.