You can use 6AU6 in place of 12AU6. They have identical characteristic, the only difference being the heater voltage, 12.6 V @ 150 mA for the 12AU6 vs 6.3 V @ 300 mA for the 6AU6.
You can compensate the difference by adding a resistor in series with the heater. Sure, it's an inefficient solution, but it works seamlessly.
Max
