Topic on Talk:Halo 2
Yup, Halo 2: Anniversary is the only copy available digitally. The original Halo 2, released in 2007, was released on retail discs. It might've also at one point been available through the Games for Windows - LIVE Marketplace, but that service shut down in 2013.
As for copies of the original retail version, there might be some available on thrift stores or other outlets specializing in old games. I picked up my copy like 2-3 years ago from one of those, anyway.