At gun shows, nearly any of the Tanfoglio/Witness mags will work in a CZ and be good quality. But, as noted in an earlier response, Mec-Gar is the way to go.
Mec-Gar builds the "factory" mags for many gun makers, and they now build nearly all of the factory CZ mags -- and they're a lot cheaper than buying them from CZ. (Used mags for CZs are generally a good deal, and you can get replacement springs from Wolff Springs.)
CDNN usually has some best deals, but the madness about hi-cap mags seems to have exhausted their inventory. Here's a link to CDNN, and that the smallest selection I've seen in years. This is a BAD TIME to be hunting mags.
http://www.cdnninvestments.com/cz.html