Try looking into guitar tube amplifier tranformers.
They usually take in 120v AC and out 6-7v volts at higher current for the tube 'heater's, along with higher voltages for the tube gain stage.
You should be able to go the other way as well.
Hmm, but you have 5VDC - and transformers run on AC.
So you need a DC-DC converter. Or do the Nixie tubes need 170V AC?
Supply some more info please.