Is 6x6.5" a standard size for a lens board? I don't recall having ever seen one.
No, how ever it's a nice large size so easy to make new lens boards or an adapter. In my case with 3 Agfa Ansco 10x8 cameras I have an adapter board to use Linhof/Wista lens boards, another to take the lens boards of my British 12"x10" field camera, and one Agfa Ansco came with a board that takes Pacemaker Graphic lens boards.
I make lens boards, adapters, etc, regularly, however I'm not touting for your business, trans-Atlantic postage is prohibitive. There are too many variables unless I have the same model camera I or any one else would actually need the camera on the bench to make something new. Having said that it is not at all difficult.
Richard Ritter would be the top end US provider for new lens boards and/or adapter in the US, But there are others selling wooden lens boards in the US on eBay.
Ian