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Also search for "nixie tube power supply" there are many circuits out there using boost converters with a 555 or MC34063 etc.

I have seen old CCFL transformers or even a backwards connected mains transformer like 6V-220V used to make HVDC.

You can use a DC-DC converter salvaged from a disposable camera (xenon flash), or buy the DC-DC converters from Aliexpress.

It depends on what you want. Easy, low cost or to learn and have fun.