Convert 5VDC to 400VDC
A MAX641 with a suitable FET, see for example this circuit of a Geiger counter (p 34/39).

Incidentally, one of my current projects is to build a Geiger counter for LEGO Mindstorms NXT with a Soviet SBM-20 tube, which needs 400V and at most 50µA.  The power supply is 4.3V at 20mA, and I plan to use a MAX641 with a BSP126 or BSP130.

There is also this thread (in german) about circuits for an SBM-20.