This basic schematic diagram of dc to dc converter shows hiw to convert 12v supply to 9v suplly.
DC to DC converter 12v-9v circuit diagram
Zener diode Z1 can be replace with 10V zener diode to get more precise output voltage. R1 also can be replace with 1k resistor.
Q1 is optional in this circuit but highly recommended.
You can replace Q1 for a more robust type to get more output amps depending on your requirements. Simple circuit to power your 9 volt cassette recorder and other stuff.
C3 = 330nF, Ceramic
Z1 = 9.1V, 1watt zener
Q1 = ECG184, NTE184
R1 = 560 ohm
C1 = 1000uF/40V, Electrolytic
C2 = 10uF/25V, Electrolytic