Something like this?
http://www.powerstream.com/mini-itx.htm
If you already have a small 120V PSU lying around, you could get a small and cheap power inverter to step up the voltage, however, the efficiency will suck.
I also believe it is fine to parallel voltage regulators. Don't quote me on this though, but I believe this is how they do it in regular power supplies, but they may also put diodes on the output.