These are the old Border Patrol type police holster that was so popular in the 60's and 70's.
Tex Shoemaker still makes several different versions for revolvers and Autos.
One uses the old over the rear sight strap and the other a more modern thumb break. These are available for most revolvers and barrel lengths:
They also sell the correct "Sam Brown" type duty belt in which the buckle is actually an adjustable double hook to fasten the belt.
Shoemaker is about the only place left who still make this type of holster, and I'd bet the "Walking Dead" company bought a Shoemaker holster.
In addition to "The Walking Dead" this type of holster also starred in Dirty Harry "Magnum Force".