Chris, no idea why this seems to be the only pace that has them, but I got mine from Ohio Airguns;
https://ohioairgun.com/products/fx-bottle-valve
Agreed - that is where I have recently purchased a couple when they had some stock a few weeks ago.
Note that in the photo in the link, it shows the older 250 bar valve. But the ones I recently purchased from Ohio Airguns were the 300 bar version (full length threads) with the M3 compatible valve pin.
Now just waiting for some to come back into stock. In the meanwhile the 2 700cc bottles I intend to use on FX guns will get the valves off the OEM bottle. The Saber Tactical Bottle Degasser kit is a godsend when doing this.