I doubt you will find one. The TL494 is a low voltage pwm controller (Vmax = 41 V). A PC power supply usually has a primary side (mains voltage) switch mode controller. A TL494 cannot be used efficiently in this application (note that it may be used with additional auxiliary circuitry, but that is not efficient neither in cost nor in power requirements).
What is the reason you're looking for a TL494 based power supply? 20 A on 12 V can be delivered by many off the shelf PC power supplies.