This website says that the last home of Mary, the mother of Jesus, was in Ephesus, Turkey.
Where did this idea originate?
Here is a Wikipedia article (poor resource sometimes, I know) that details the origin of the story that Mary lived in Ephesus (or nearby).
It appears to start with a nun in Germany named Anne Catherine Emmerich, who was bedridden and reported a series of visions that detailed "the last days of the life of Jesus, and details of the life of Mary, his mother." She had a visitor, who was an author named Clemens Brentano, who transcribed her visions and later after her death published a book about them. In one of his accounts was a description of a house that John built for Mary in Ephesus, which provided details of the area:
"Mary did not live in Ephesus itself, but in the country near it. ... Mary's dwelling was on a hill to the left of the road from Jerusalem, some three and half hours from Ephesus. This hill slopes steeply towards Ephesus; the city, as one approaches it from the south east seems to lie on rising ground.... Narrow paths lead southwards to a hill near the top of which is an uneven plateau, some half hour's journey."
In 1881 a French priest, the Abbé Julien Gouyet, found the house by using the notes in Brentano's transcripts. The rest, as cheesy as it may sound, is history.
There seems to be only two historical possibilities as to where Mary ended the pilgrim journey on earth: Jerusalem or Ephesus?
Historically, there are proponents to both sides of this question as can be seen in this article on the Tomb of the Blessed Virgin Mary. These two theories do not seem to take into account the solicitude that St. John would have had in keeping Mary safe! We know that Christians were being persecuted at Jerusalem from the very foundation of the Church, so Ephesus is a real possibility.
"Catholic and Orthodox tradition say that John, together with the the Virgin Mary, moved to Ephesus, where both eventually died. According to Tertullian, John was banished (presumably to Patmos) after being plunged into boiling oil in Rome and miraculously suffering nothing from it. Some believe his tomb is located at Selçuk, a small town in the vicinity of Ephesus.
In his Dialogue with Trypho (Chapter 81) Justin Martyr refers to "John, one of the Apostles of Christ" as an eye-witness of Jesus' ministry who had lived "with us" at Ephesus. Irenæus declares that he wrote his Gospel at Ephesus (Adv. haer., III, i, 1), and that he had lived there until the reign of Trajan. Eusebius and Jerome related that John was the teacher of the Papias, the bishop of Hierapolis in Syria. When John was old he is believed to have trained the future Saint Polycarp, bishop of Smyrna, in today's Turkey. Polycarp is thought to have carried John's message and apostolic authority to a new generation, until he himself became a martyr for the faith."
The visions of Catherine Emmerich seem to be a confirmation of the Ephesian tradition, rather than its' source.