Unfortunately they're pretty much impossible to find now a days. The originals were a custom one-off pair from Airwalk specifically made for The Matrix. A few months after the movie hit theaters, Airwalk did a special release of the boots, limited to 500 pairs, which retailed for $165.
As far as I know, MotorCowboy is the only place you can now score these boots. Keep in mind, they aren't "official" Airwalks, they're replicas. They look pretty spot on, and I hear MotorCowboy's products are solid quality, unfortunately they're pretty pricy.
http://motorcowboy.com/products/matrix-boots-worn-by-neo.html