You can't buy it in a comic book shop because it's not a traditional comic; it's a project which has been tailored specifically to be enjoyed on wireless devices
First one's free, cheap after that. Full-color digest edition available in print format when the run's done. From an interview with de Campi:
The thing you also need to keep in mind is that comics overseas are far, far bigger than they are in America. In France and Japan, there are single issues of a bande dessinee or a manga tankubon that regularly outsell in volume the entire US comic industry's output for the year.
And the format thing? Well, frankly, that's just showing off. (No, seriously, as I was talking to people about "Valentine," everyone was like, "Oh, I read on Stanza, can you have it for Stanza?" and "Could I get it on my Kindle?" and "But, I have a Sony e-Reader"…) and the joy of doing one panel per screen is that it makes the format very adaptable, both for different size screens and for right to left languages. One panel per screen may not be the way of the future, as technology evolves on an almost moment by moment basis, but it has worked very well so far for "Valentine."