Cossacks II: Napoleonic Wars and its add-on are now only available digitally from the following websites. For the boxed versions, check out Amazon, Ebay, etc.
Cossacks II: Napoleonic Wars
Cossacks: Napoleonic Wars can be purchased separately exclusively on Steam at:
Cossacks II: Battle for Europe
Cossacks: Battle for Europe can be purchased separately exclusively on Steam at:
Cossacks II Anthology
You can get this pack from GOG.com. It has the advantage of being completely DRM-free, but the multiplayer mode is only available with the CD key that comes with the games, which can be requested separately from the retailer’s website.
Other bundle
Cossacks II: Napoleonic Wars and its add-on are available at a significant discount through Steam in a bundle called “Cossacks and American Conquest Pack”.