The official Delta Green site is mostly focused on modern-day stuff, and includes some adventure seeds and random background stuff. Nothing that seems very relevant for us.
Gil Treviso has a campaign site up, again mostly focused on a modern campaign, but including a detailed history of the Office of Naval Intelligence. There is also a bunch of material on the history of P Division, which I haven't really looked at as (a) I'm not sure how much Jon wants us to know yet, and (b) I don't know if it even corresponds to Jon's world.
And there's a Delta Green Web Ring with a dozen links. I haven't looked at any of them yet.