As already mentioned buying a complete hub ensures that everything will match. The obvious donor is an WH-7700/7701 since they use the same FH body.
But there are a few pieces to this. If you look to mix and match you want the cone to be similar, (I'm not an authority on grind differences), and the dust seal to work. The FH-7700 seal is 214-0600. The FH-7700 DS cone dims are M10x13.2mm. The Ultegra FH-6500 uses the same 214-0600 seal and the DS cone dims are M10x13.5mm. Now finding an FH-6500 might be equally difficult.
You can get aftermarket cones from Wheels Mfg, but they don't offer one for you hub.
https://wheelsmfg.com/products/hub-p...=1711224210326
John