I usually just make my own. Hope that helps. I am looking for an inexpensive source for early appearing keys. I am a lover of old keys. Having a real house at Pennsic made it appropriate for me to have some keys dangling from my belt.
Flea markets are another good source.
The wards of the key, to appear more medieval, should be like a maze, with intricate cut outs of a square field. The ring should be round. But most people won't know the difference today. 12th and the 19th had round bows because tehy wer the simplest to make. Some were diamond shaped, some had pieces that were cast, and had all sorts of fancy scroll work, etc.