This trophy can be won by Chapter IV, Plane-Wrecked. If you haven't missed any prior to this, you will find your 15th treasure midway through the chapter.
Gamefaqs user YYZed has put together a comprehensive guide:
http://www.gamefaqs.com/ps3/932984-uncharted-drakes-fortune/...Youtube user dragoszontara has a video guide as well: