Neon Series resistors
The specs for neon lamps will vary from type to type but in general they require something between 0.25mA and 0.6mA.
These devices are CURRENT operated, Not VOLTAGE -- Beware :twisted: I agree they will require a high (!) voltage to strike (value depends on type), but they need a much lower voltage to run :shock:
If there is no current limiter in the circuit they will go --B-A-N-G--; the current will reach some scary value and will blow a 5Amp fuse easily.
I would expect a 220K resistor to be about right for the proposed application.
As an aside ...
Old electric organs used neons for the oscillators -- charge a small capacitor (0.1uF) from 100V DC via a large resistor (1M0) and the neon (connected in parallel with the cap) will flash at a steady rate, producing a sawtooth waveform. This is a relaxation oscillator.
A similar circuit; when one lead is hand-held and the other lead offered towards the emitter, will test an electrostatic ioniser. The flash rate depeding on distance from the ioniser and its emitter's efficiency. 8)