Testing a CW source for voltage versus current

I have a Cockroft-Walton voltage source that I would like to test under load. The output is about 25kV DC at zero load and I would like to test this under variable current loads from 0-30 mA. Can anyone offer suggestions as to how best to apply loading with a known value of the current? I realize I could buy some huge resistors, but ones with the right specs (power, voltage) seem to be unavailable or very expensive. Is there a better way?

Thanks
Eric

I thought I have the machine you need. I just found it. But it is not quite what you want. Up to 27kv 0 to 3mA. (high voltage power supply tester)

It uses 6BK4 tubes as a load. I also have some 30kV resistors but switching the resistors in/out is a problem.
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wow 750 watts!

The idea of the 6BK4 tubes is interesting. Any way to use a plasma tube for this?

The 6KB4 is good for a variable current. I have the schematic for the tester. There aren't may tubes that will take on 25kv.

I don't know of a good way of switching resistors.
I do know how to make a 1mA load, a 10mA load and a 30mA load and you pick the one you want. (turn off the power to switch resistors)