Elmore Leonard, who wrote the books that "Justified" is based on, also has a bunch of good Westerns, many of which became movies. A personal favorite, not quite a Western, is "The Hot Kid', which has a lot of gunplay and seems to me to be a precursor to the Raylan Givens stories.
"Mountain Man", by Vardis Fisher, became "Jeremiah Johnson"
"The Big Sky" series by A.B. Guthrie
"Welcome to Hard Times"
"The Last Hunt"
