Do you really need 500V rated ones ? If so these mil spec ones are pretty good. I have bought parts from this crowd and not received any fake items, they are certainly cheap enough:
https://www.ebay.co.uk/itm/1pcs-2N67...oAAOSwstxVXGDG
If 400V rating will do, go for these lovely ones from Harris:
https://www.ebay.co.uk/itm/2N6760-HA...cAAOSwx6pYq4y1
I have used a lot of Harris TO-3 fets , always wonderful
These ones also look genuine:
https://www.ebay.co.uk/itm/JANTX-2N6...EAAOSwl9BWLDGA