What do you mean by "input voltage"? Supply voltage or as your question suggests the input to the trigger pin?
For the supply voltage:
Resistors alone will not work properly. Due to changes in the operating current the voltage drop will vary. You'll need some kind of stabilization. A simple
zener diode stabilizer can do the job. Add a capacitor of 10 µF or more at the output of the stabilizer to smooth the voltage and supply impulse current.
This is a linear type of regulator and as all these regulators it is very inefficient. The series resistor has to drop ~290 V (assuming 300 V input, 10 V output). At a moderately small current of 10 mA this means a power dissipation of 2.9 W in the series resistor.
A more efficient way would be using a switched mode step down regulator, but you'll be hard pressed to find a small and affordable one for this application.
The most cost effective solution would be to use a small wall wart with 5 V to 12 V output voltage.
For the trigger input:
A voltage divider can be used to scale down the 300 V input voltage to a level compatible with the supply voltage of the 555. As an additional protection a zener diode across the trigger input can be used to limit voltage spikes.