Most of the outside cabinets are the CH751 key.

https://www.amazon.com/dp/B01A97WB5K/ref=cm_sw_r_awdo_navT_g_K23W9BN1PWPZKK2PCVRY

Unless you're stuck somewhere with a flat, see if you can look up what key number was used. It's probably that one, but chances are whatever it is you can order one.