Szlachta
Some powerful Polish nobles were referred to as "magnates" (Polish singular: "magnat", plural: "magnaci") and "mozny" ("magnate", "oligarch"; plural: "mozni"); see Magnates of Poland and Lithuania.
What other country  besides Poland were polish nobles most likely found?
Lithuania
Some powerful Polish nobles were referred to as "magnates" (Polish singular: "magnat", plural: "magnaci") and "mozny" ("magnate", "oligarch"; plural: "mozni"); see Magnates of Poland and Lithuania.
Poland