Tektronix P6015. You don't need to fill it with the dielectric Freon for 10kV.
https://www.ebay.com/itm/387679298575
the same seller has some cheaper kits without the freon bottle.
about 10 years ago i recognized the core HV probe guts sold under the military NIN number and bought it for a few dollars, then built my own compensation box using the schematic. it works ok.
yeah for high energy stuff you want to test your probes to at least 50% more voltage than what you're dealing with. the derate with high frequency is no joke, although i don't know if that's due to potential breakdown or just heat dissipation.
the 6015 has a neon bulb in the circuit to limit the voltage that could get to the oscope to just a few hundred volts. being 1000:1 the most OP should ever get is 10 volts at the oscope.
115$ for all the stock parts is a good deal.
https://www.ebay.com/itm/135145449204my guess is that you could fill it with R152a (canned air duster) and the stock plastic components will have no problem handing the pressure at room temperature.