Both of Chris Kyle's books, "American Sniper" and "American Gun," are available for purchase online. The books can be purchased on Amazon or Barnes & Noble. They are also available as audiobooks on Audible.
Chris Kyle's website links directly to the Amazon pages where you can purchase both of his books. "American Sniper" sells for $6 in paperback, while "American Gun" retails for $10 in paperback and $23.50 in hardcover, as of 2015. Barnes & Noble sells "American Sniper" for $10.
On Audible, the audiobook of "American Sniper" costs $23.50. Both books are also available in Kindle e-reader editions, which can be purchased through Amazon.