Your best bet is to watch the auction sites for a Dirty Harry type holster. They come up every so often. You can find them made by Clark, Bucheimer, Cathey and others.
The holster used in the movie has a narrow retention strap, sewn holster to yoke, and a tension screw. You can find variations with a wide "cobra head" retention strap, laced holster to yoke, no tension screw, and even some with an elastic harness.
This is the narrow retention strap like the movie holster.
Here's the "cobra" style retention strap found on later style holsters.
This holster has the sewn harness like the movie holster.
This holster has the laced harness found on many Dirty Harry style holsters.
Something else to consider is that Harry Callahan was wearing a Bianchi Phantom in Sudden Impact. The holster can be clearly seen in the scene where Harry is practicing with his 44 Auto Mag.