As I discuss in this answer, the oldest Vishnu statue in the world, and probably the oldest Hindu statue in the world, is the famous Ranganatha statue in Sri Rangam. The story of Ranganatha goes roughly as follows.
Shortly after the birth of Brahma, which I discuss in this answer, Brahma started praying to Vishnu (perhaps with either this hymn or this hymn). As a result, Vishnu appeared before him and gave Brahma a divine statue of Vishnu lying down on the serpent Adiseshan. Much later, Surya the sun god began to worship it, and then Surya's grandson Ikshvaku had the statue brought to Ayodhya so that he could continue worship it. The statue was passed down through the kings of Ayodhya, until it came to the possession of Rama. Rama gave it to Ravana's brother Vibhishana, who planned to install it in Lanka. But on his journey from Ayodhya to Lanka, Vibhishana put the statue down in the island of Sri Rangam in the middle of the Kaveri river, so that he could bathe in the holy Kaveri. When he came back, he found that he could not pick up the statue with all his might - Vishnu wanted to stay in Sri Rangam. And that is where the statue remains to this day, in the illustrious Ranganatha temple of Sri Rangam.
But my question isn't about the story itself, but rather about the source of the story. The story is found in the Koil Olugu, the temple records of the Sri Rangam temple which you can read (in abridged form) here. But my question is, what Hindu scriptures describe the story of Ranganatha?
I've seen claims that the story of Ranganatha occurs in two Puranas:
The Sri Ranga Mahatmya of the Brahmanda Purana, also known as the Dasadhyayi or "ten-chapter section". This Telegu work by the 18th century king Vijayarangachokkanatha claims to be a translation of the Dasadhyayi.
The Sri Ranga Mahatmya of the Garuda Purana, also known as the Shatadhyayi or "hundred-chapter section" (although it's said to contain 108 chapters).
Yet I can't seem to find the Sri Ranga Mahatmya in either one of these Puranas; in my answer here, I provided links to all five volumes of the Brahmanda Purana. I've also checked the Garuda Purana, and I can't find either the Dasadhyayi or the Satadhyayi in these Puranas. Does anyone know where they are?