300 Volts at 50 Amperes is 15kW of power - a very large industrial machine. Can get away with a tenth of that if cutting speed isn't a huge concern.
Such a power supply would be very large (and costly) and consist of a three-phase 480 VAC input. This would be bridge-rectified to ~700 VDC, which is of course very dangerous. One or more 300V adjustable bucking DC regulator(s) working in synchronicity would drop this to the target voltage. Many bulk energy storage capacitors would be used to keep this level constant. And some fast means of switching these large currents on and off (MOSFET or IGBT) would be used. Each pulse would have very fast (tens of MHz) current-loop monitoring, to ensure the current stays where it should during each pulse, and doesn't blow anything up.
You won't find such a schematic because the only team of people whom have them, spent years and a whole lot of money fine-tuning a niche device for a niche application. They won't share them, ever.
The supply of power isn't really all that complicated; the power just burns away the metal. A welder can burn away metal. But EDM is a rather unique use-case for a power supply; special considerations must be made such as what to do when pulse current gets too low (should the voltage increase?) or too high (reduce voltage or stop machine?) And should current stability be prioritized over voltage stability? And in the case of wire EDM, too much current can break the wire; how to handle that? All of this must be analyzed on a pulse-by-pulse basis, including sufficient dead-time for the fluid to flush out the swarf. Then a whole host of ancillary machine concerns arise, such as fluid conductivity monitoring / filtering, temperature regulation, quill/wire feed control, etc.
Anyways the "cheap EDM machines" you can find on eBay and Aliexpress are just that - cheap. Very cheap. They will output some volts and some current, and could be used in a pinch to EDM-out a hardened bolt a few times before they die. Inside they use a primitive voltage and current regulator (primitive like a single big capacitor, inductor, and resistor.) They have none of the engineering design provisions for robustness, protection, optimization, and feedback control that a 15kW machine would require.
Suggest researching the BAX EDM website and videos for more details.