0-1000V Regulated High Voltage
Power Supply by IC 555 , LM339 , 7805

Another power supply that can be used for the Geiger-Müller tube variant of the cosmic ray muon detector.
If the specific transformer mentioned in the schematic is not available to you, any transformer with the specifications primary 117VAC, secondary 6.3VAC CT (center-tapped) should work.
In this case you might have to choose a different drive frequency in order to operate at the sweet spot of the transformer: "​[...] I found in driving a transformer as is done in this project that the overall performance varied significantly as I varied the drive frequency. In a series of experiments I loaded the transformer directly with a variable resistor (no rectifier used), measured the AC voltage across this resistor, measured the DC current into the transformer, and varied the drive frequency. I looked for a frequency where the output voltage was highest and the input current was lowest. [...] 'ripple' is roughly +/-25V for output voltage >200V. This ripple isn't sinusoidal at all. It is basically a spike then monotonically decays until the next spike."

Read More Source:
http://www.cosmicrays.org/muon-power-supply.php
Thank you.