There's nothing to find. You need to create the Portraits folder yourself, and then put .bmp-files in there to use for portraits.
Getting .bmp-files to use for portraits is pretty easy. Just Google "Baldur's Gate custom portraits" and you should get at least 3 or 4 sites that will supply you with good custom portraits of the right size for both the small and the large portrait slots.
Most of these portraits are scans from D&D artwork books. I own one such book and have spotted a few portraits that people have scanned an then cut out of larger artworks from it.