Have you considered an electrophoresis power supply, I have a biorad 500/200, this is a variable linear psu, as the tag suggests 500v at 200ma adjustable, ideal for bench testing, all you will need is a supply for the heaters.
"One possibility that I am considering is building a power supply for an Allwave 23 chassis that I have.
I bought the chassis as a parts chassis without a power supply. While I do have an original supply that goes with my complete all wave 23, I…"
"Unattached amps do turn up occasionally - but they aren't easy to find. I know there was a 23 amp on ebay just a couple weeks ago, keep your eyes open. I do not have any spares, as I said, they can be hard to locate.