Trying to find top bins for CXA sounds almost like a fools game.
I haven't looked very hard, but on digikey for 3000k 80cri, I can only seem to find Z2 and Z4, which are pretty expensive already. The reason you can't find them cheaply has nothing to do with whether you get 1 or an entire reel. It has to do with the nature of binning and the distribution of cobs produced in the highest bin being so sparse.
Only using AB cobs would be analogous to only hiring workers over 130 IQ. If every employer did this, only 2% of the population would be employed.