You can get the ignition wire from the harness where your radio used to go. It's most likely a red wire.
I think IGN wire is used only to trigger DCDC-USB to start drawing power from the battery, so there's no heavy draw on IGN itself. I couldn't tell you for sure, however.
If DCDC-USB indeed does not draw that much power from IGN and uses it for triggers only, then I don't see why you couldn't use a cigarette lighter power, as long as your car powers it down. Some cars keep the cigarette plugs on even when the car is off, so I'd verify your car doesn't do that first.