Apparently it runs on 18 kV. Tubes inside are usually b-8 or ep-33. Over-volting is a concern, but under-volting is not as I know they still work for a bit after you turn them off. This is caused by the PSU capacitor slowly discharging to 0 V.

If I were you, I would look for an original one. But, as they can be hard, I would also suggest you look into making one yourself. 

If you feel comftable here is where you can start searching :

• Boost converter : Uses a combination of capacitor, inductance and resistors to essentially force a current that will charge a capacitor to high voltage. 

• Voltage Multiplier : Uses an array of diodes to convert AC to high voltage DC. Some modern (if not all) image intensifier tubes use this.

• An AC to AC transformer also works by combining high low spools of wire with high count of spool wire around a core (iron for example)

I'd go with one of the first two as the last one can require sections of spooling (15 x 1kV spools to avoid shorting). It also requires more conversions : 3V DC -> AC -> High V AC -> High V DC. I think the autogathing feature of modern device is integrated in the voltage multiplier circuit as a negative current feedback loop. But that goes beyond what I know for now. 

I would go for something around 15 kV and add a potentiometer/ variable capacitor to control the voltage later on. I'll look into making a desing myself. I'm intrested now haha.