The fastest way is to get jonin medal, 30 frags upon first activation. Then, as others mentioned: some events have rewards on hih participation like lucky sntch, sometimes naruto frags are there. Many events like shuriken or time limited ninja recruitment (1 point for 10gold spent elsewhere) have his frags as well, they are usually around 800cp 1. On a rare basis you may encounter him during chase missions and thats propably all of the viable ways.