I feel sort of bad for anybody looking to run a Delta Green d20 campaign. I got the Call of Cthulhu d20 book when it came out, but it's going to be pretty hard to pick up nowadays isn't it?
I haven't used d20 modern, so I don't know if it'd work as a suitable substitute. Call of Cthulhu has always been about the stories and adventures to me, so as long as you find a system you like, it should work fine. If I had the time, I'd convert my upcoming DG campaign to GURPS.
Anyways, to be helpful to any potential DG GMs, there's also Delta Green: Countdown (Which isn't in print, but still in stock in many online shops) which is an even huger sourcebook you'll want to pick up. No d20 stats, but it'd not be hard to convert (especially since most of the material is fluff).
Secondly, there's a compilation of the long out of print Delta Green chapbooks being released this year.
http://web.mac.com/drgonzo/iWeb/Site/DGEO.html I didn't have the privilege of having those when originally issued, but more Delta Green can only be good.
If you really get into Delta Green, looking at some generic Call of Cthulhu Now! adventures, such as Unseen Masters and The Stars are Right. Many of them work wonderfully as DG adventures.