I am just guessing but 1pf is approx 1-2cm of dangling wire. 15pf capacitors are rather easy to get and they will have perhaps 100-250v working rating.
In the circuit you present, the capacitors get the same voltage as the resistors. If each resistor is getting 250v, the total applied voltage is 2.5kv. You will need capacitors with at least 350v rating.
The layout will be important in this particular case. Also take usual precautions when using high voltages.