What you need is another "power supply" mainly a small stepdown transformer from 120 12 V AC. Go through a ful wave bridge rectifier and a regulator (7812) for your 12V needs, and through a 5 V regulator (7805) for your 5 volt needs. All this is assuming you need less than 1Amp of low voltage DC. If you need more at 12V then you would have to size that part of the power supply for the 12V needs and still use the 7805 for the 5 V needs. A few appropriate Capacitors at each stage of the DC would clean it up and make the devices connected happy.