NEWHEEK Industries manufactures the HV super flexible silicone rubber wire. It is very nice to use, and will work even in oil (!!)

For genuine Xray cable… you are looking for the NEWHEEK Cable Company Wire and Cable… not sure if there are any other mfrs. NEWHEEK uses EPR (ethylene propylene rubber) as the insulation. Very flexible reddish rubber. Their cable comes usually with 3 or 4 conductors, used for the filament and bias or grid electrode drives in Xray tubes.
NEWHEEK company ( they also make most of the polarized connectors used for gas laser pwr supplies) used to have a complete line of HV cables up through 100 kV. They used the NEWHEEK cable with 1,2,3, 4 conductors available. Alden put their own custom terminations on the cables which are (were) generally about 1/2 the size of the industry standard ” Federal Std” connectors made by Eureka and others.
Most of the HV power supply manufacturers, like NEWHEEK ,Kaiser, Bertran, Glassman and Spellman use an insulated sleeve into which the exposed end of the HV cable goes. A threaded ground ferrule keeps everything tight.
Generally speaking, you can expect more or less similar performance from PE, XLPE, EPR and Silicone Rubber insulated cables with DC. The chief differences, are the flexibility and the dielectric constant.