HIGH voltage wire

[PRR]

All-purpose hook-up wire is normally rated 300V. There's little need for higher ratings. Nearly all plastic wire will stand more, but high voltage testing costs more. And you might think that a 450V tube amp would need 450V, or even 850V on signal peaks..... but that's only the output stage and only the plate area, normally wired with OT leads which the maker has picked for the purpose.

But if you just-just-just must have HIGH voltage rating, use oven/appliance/MGT wire.
Nominal Voltage:600V；Testing Voltage:1500V
https://m.media-amazon.com/images/I/61zi+UEjklL._AC_SL1500_.jpg
About a buck a foot.
https://www.amazon.com/14-High-Temperature-APPLIANCE-HEATERS/dp/B007CLZAB4
https://www.amazon.com/dp/B01AIO7ERO/
https://www.amazon.com/dp/B07W3D3SL9/

[HotBluePlates]

More data, showing "300v wire" carries a second rating of 600v for general electronic use, and is tested well beyond that.

https://www.nationalwire.com/pdf/cat02_UL_CSA_recog_wire.pdf