This particular one here is the Amtherm MS35 20010 and can be bought from Digikey (570-1026-ND). Its nominal resistance is 20 ohms, and as temperature rises, it drops to very low resistance.
Now you might say "Great, just put this baby in series with my AC input line and I am done". In some cases you might be able to do this but the power amplifier I am designing is a 200W per channel set up and, though the pyrotechnics might be fun to some, being nominally averse to smoke and crispy electronic components, as well as slightly paranoid, I decided to add a relay to the circuit.
In addition, when someone turns off a device that has been on for a while, the thermistor will be very hot. If they then turn on the device immediately after shutting it off, the thermistor will dutifully say "I'm hot and tired, and I am not providing you any resistance". Your transformer will say "Ow" once again.