Apart from the insane price of A172 CUP and C172 CUP I have never seen "X172 XXX" plates for sale on the various plate sites. Yet driving to work this morning I followed a C172 XXX and then a T182 XXX (just on vans). Im guessing as they are standard format plates they are deemed not worthy of selling by the DVLA etc. Can you get them anywhere?
Im sure someone is pootling round in C182 CUP somewhere, probably in a car worth less than the plate!