Photographers like to crop too closely for portrait work, but I've found lots of fodder at
www.photo.net.
If you're really smitten with a photo, you could try emailing the photographer before you even start. If you have permission in advance, you'll eliminate lots of problems.
Personally, I draw from found photos all the time, but I consider it strictly a practice resource. I've seen artwork for sale on ebay and on artists websites that were made from photo references I was familiar with. To me, this cheapens the artist in my eyes, and makes me question the originality of his whole portfolio, not just the ones I recognize.