Any old flyback will do, here's one for something like 100-800V and 50W:

The schematic is uninteresting, a UC3843 flyback circuit. Check the datasheet / appnote for typical use.
- The two transistors are in parallel, just because I didn't have a single part rated for the purpose. (Or, actually, they don't really get hot, I probably could've used just the one, with that big heatsink on there...)
- Drain voltage rate snubber, FR302 || 47R + 4.7nF to GND.
- Stack of caps on the primary side is just for bypass, 4 x 220uF 50V.
- 1.5KE15A clamps controller supply, just in case of overvoltage/inrush/surge. Not needed, just nice to have in some cases.
- Transformer is wound with foil primary, secondary is dual, 28AWG. Interleaved PSPSP. Gapped EE33 core (Ae = 100mm^2). Diagram:
https://www.seventransistorlabs.com/Images/DCDC_800V_FoilWindup.jpg- Secondaries are independent, half-wave rectified each, and stacked in series. Effectively it's a dual 400V supply. One side is grounded, hence the output is up to +800V.
- 2 x 0.68uF film caps and 2 x 10uF 450V electrolytics bypass the output. UF4007 rectifiers. 100uH choke to 2 x 0.47uF film caps provides additional filtering.
- 200k bleeder keeps output safer to handle when deenergized, and sense divider is 1M--20k trimpot--3k to GND.
Tim