"Gunfight" is a great one to start with.
As you can see from IMDb, Kelley was mostly a TV actor, and predominantly in Westerns. One of his films I am familiar with is Warlock
, an excellent movie and a very good example of the '50s "psychological Western" directed by Edward Dmytryk. Henry Fonda plays a mercenary gunman who's hired to protect the town of Warlock from a gang, Anthony Quinn is his partner in sort of a Wyatt Earp-Doc Holiday dynamic, and Richard Widmark is a good guy caught in the middle of everything. Kelley is a member of the bad guy gang, not a very big role but fairly important at one point. TOS guests Frank Gorshin and (big surprise!) Whit Bissell also appear, and if you pay attention you will see Gary Lockwood in the gang as well.