Crisis is for sale, although it tends to be sold out a lot. I picked up a copy of base game and all the Crisis packs a while back. It can be a lot of fun to play and is easily learned.
I have only one set (and all Crisis packs) so I haven't experienced the 4 player variant yet but 2 player is fun enough.
The digital version for PC and tablets, base game only, also has quite a few people playing it. There are two options there, one with five minutes per turn, and one with 72 hours per turn for when you just want a leisurely strategy game.
There's another expansion (Gambit), but I'm not too sure what that is. The site has little information on it and I can only find conflicting data. It's either the Kickstarter bonus, a tournament set, or both... I'm not getting it for now at least. Last I heard Gambit would get an official release some time in 2015.