If you are interested in finding antique door knockers you should find that it is fairly simple to locate some, especially those dating back to the Renaissance. In fact, some door knockers made during the early Macedonian times have been found however these are naturally extremely rare items.
Door knockers have been a common sight on the homes of many since the early Roman days. Originally only the nobles or elite could afford to have these items made. This is true even through medieval and Renaissance days mainly due to there being a lot of craftsmanship involved in their making. In those days an artist was commissioned to design the actual door knocker. Once the design was satisfactory this would then be taken to a metal smith who would then begin the sometimes laborious task of actually crafting the item from the desired metal.
From around the middle of the 19th century all the way until the early 1900s there were not a lot of door knockers made. During this time a more modern approach was taken towards decorating homes. Old heavy doors were taken down and replaced by much lighter doors and in general many items were designed in a much simpler manner -- that seemed to be the preference at the time.
Sometime in the early 1900s there was a renewed interest in door knockers and other types of antique or old-style furniture and accessories. Those who had tucked away their old-fashioned door knockers were relieved to find that they were once again in vogue. Since that time they have been used both for decorative purposes and, prior to the invention of the doorbell, also for practical reasons.
It is fairly easy to find antique door knockers online although you do need to be careful as there are also a lot of replicas which are often so well made that it can be difficult to tell the difference. Of course you can also often find them at flea markets, garage sales and in secondhand stores. You will also often be able to locate them went rummaging through storage in old houses as often times they were taken down when a new doorbell was installed but kept in storage.
Whether your interest is in collecting genuine antiques, or you are just looking for an antique door knocker to add to your front door, you should not have much trouble finding one. In the event that you are not able to find one and you merely want one for decorative reasons you could always consider purchasing one on the replicated medieval or Renaissance door knockers that are available.