If you are looking for antique door knockers, you may be able to find some that go way back, because door knocker, or rings to open the doors, have been in use since the Renaissance at least. Whether you can find something that old is problematic, as metal gets recycled, but you should be able to find antique door knockers at least 100 years old. Talk to your local antique dealers and let them know you are interested, and you probably hear from them fairly soon.
The antique door knockers may be made of iron, or may be brass door knockers. The brass door knocker may not impress you, as it will be oxidized, but talk to tan antique dealer before you clean it. It may lose value if you clean it, many metal objects do. If you really want a shine brass door knocker, but a new Baldwin door knocker and collect the antique ones as objects of interest instead of objects intended to be used.
Antique door knockers will have many forms, including the lion door knocker and the doctor door knocker. The first establishes the lineage of the owner, while the second let people know the profession of the door owner. Other door knockers come in various other motifs, like anchors or sailing ships for the nautical, woodpeckers for the naturalist and other shapes. Generic door knockers of the past include a hand holding a ball as the striker, or a plain ball. Some door knockers are one piece, but there are other that the striker and the strike plate are separate.
For new door knockers, the current favorite maker is Baldwin door knockers. This company produces a number of shape in several metals or metallic finishes. You should be able to find a door knocker to match you door hardware.