Your only real chance of finding a "cheap" ZF box is to get really lucks, or find one that needs rebuilding. Maybe try searching for Pantera parts and you might get lucky.
Ricardos are not made anymore, but they turn up from time to time, and are pricy. Expect ~$15k
RCR/SL (makers of the SLC) use several tranny combos. not just Porsce. Audis, Porsce, Ricardo, RBT/ZF, Graz
RBT is making new versions of the ZF as well as some variants. Also pricy and with long wait times.