It is available either from 1CGS (the developers) website or via Steam. You may also be able to order it directly from Jason at 777 Studios who is a partner in the development of it, Battle of Moscow and Rise of Flight. 777 is based in California.
I purchased both the "Battle of" titles in beta and am very happy with them. Whilst this is only my opinion and others are sure to disagree, I think the latest incarnation of IL-2 from 777/1CGS is the most faithfully modelled combat flight sim currently available and has great continued potential for expansion to other theatres in time. The shipping single player experience leaves a bit to be desired in my opinion. But the multiplayer side of the game is excellent with quite a few mission makers coming up with new and innovative ways to build dynamic missions with interest for bomber and ground support pilots as well as knuckleheads. There is also a popular 3rd party campaign generator developed by Pat Wilson with financial support from 777 studios.
I recommend it to you. It is currently on sale at 50% of the regular price.