As you have already discovered, panels are not available. The reason is, the panel is the majority of the cost of the TV and to pay for a replacement is not worth the money. For just a little more, you can get a newer TV with warranty.

What you CAN do is find another 75X850E that has bad boards but the screen is good. Then swap all the guts over to the TV with the good panel. The only problem is finding one for sale (at a cheap enough price), it could take awhile. And you also run the risk of getting one that was suppose to have a good panel but then find it wasn't.