If you scour your house and read each plug you find, you should be able to come up with at-least 1 useless 12V DC plug. Cordless phones often use 12V DC plugs. Some cell phones. old routers. Trust me, look around at your junk stuff.
I don't use a PC transformer because it creates too much heat and I don't think I need a 3-prong plug on such low power devices.
I think my 12V DC adapter came from an old camcorder.