When a neighbor ask my cabinetmaker husband to make her a fairy door, it was our first introduction to the concept. I had no idea that such things existed.
Kelfae (not sure of the artist's name) makes a polymer clay version of fairy doors. Her web site is a bit challenging but fun to browse in this season of elves and fairies. (In case you can't read the text, here's a bit of it. "Each door is individually sculpted and painted using sculpey and acrylic. Prices range from $25 AUD for a single-story, to $35 for a double story. Windows are also available. Doors are ONE PIECE, the doors do not open upon their frames.") And here's a bit of an explanation of the concept.
The link was sent in by Geraldine Newfry whose site is always a treat as well.