Can I power a photomultiplier with a camera flash power supply?
Gain of PMTs is quite dependent on supply voltage. I'd guess a flash supply is poorly regulated. If you're going to the trouble of using a PMT, then perhaps you're concerned with high, constant sensitivity. Adding a regulator to such high voltage isn't trivial, but highly recommended.

Current required should be under 5mA. You only need to supply tube current plus current of the resistor series string that establishes dynode voltages.