PC Power supply got over current protection so it will turn off before frying your Maslow CNC shield 
Go for something 100–>150W. (FUN FACT)Another problem is low temperature. They wont work if there is -10oC so pick hair dryier or use vacum cleaner fromy cyclone to heat it up
You may need somthing that can output 6Amps, so 6A*12V= 72W. So 100W PC power supply can fit. They are cheap and easy to get.