Where to find Z rank equipment

I find only A ranked equipment

You cannot find any equipment that are A ranked, they start out as E, D and sometimes C. Any 3 of these originally ranked equipment are not guaranteed any rank, beyond that E ranks cannot reach Z rank. For example, C ranks may reach Z rank, but there is also a chance they only reach B rank even after all 3 slots are unlocked. It is all based on luck, obviously with higher ranked equips at slot 1 having a greater chance, but even then it is only a chance.