
Release version of the game will contain only the state of California. So originally planned.
However, the developers already have fully prepared state of Nevada, it remains only to add it to the game. However, the developers were not sure whether to add one more state (twice increasing the size of the game world) into release version. So that version of the game, which will be able to see on the shelves February 3rd will contain only California, but do not be upset ahead of time. DLS Nevada will be free for everyone who bought the game, no matter how and when the game was purchased. The update will be absolutely free to download after the official release of American Truck Simulator.

To worry about where to buy the game not worth it. No matter whether it's Steam or any other online store, or even such a rarity as a real store somewhere in your city - all DLS content, referred to above, will be equally accessible to everyone.
Incidentally, it is worth mentioning that some retail stores will offer an expanded (and more expensive) version of American Truck Simulator, calling it a Collector's version (Collector's Edition). Big nice box, with lots of different Souvenirs, can be, for example, a great gift for a birthday, but as for the game itself, it will be exactly the same and with the same free access to the DLS, like regular version of the game.
The recommended price for the game is $20, but, of course, different exchange rates, the policy of local retailers and other factors will lead to a small variation of this price.

It would be incredibly cool to be able to get to the East coast, to head North, up to 60 parallel and to the South up to the equator. But in the end, all this is in the hands of the fans and their wallets. After all, SCS Software has repeatedly stated that their game to a large degree do fans, not themselves.