First, you must have the Foreman job unlocked for Kasuga (Charisma Level 3 is needed). Switch to Foreman at Hello Work to obtain the Demolish environment skill. There are 2 secret Black Market shops, one in the Commercial District and another by Restaurant Row near Isezaki Road (view map image). Reveal them with the Demolish skill. Gold Safe Keys cost ¥10,000. Stock up!