As far as celebrities go, I usually find them when I'm watching something. It's unlikely to be the main actor in a show/movie, but I'll see some supporting actor or cameo person and go, 'gee, they'd be great for such-and-such'. Then I find who it is and grab their image. I have a folder full of potential avatar images from various actors. 

Others have mentioned good sites for non-celebrities. 

The only model site I've ever peeked on is New Faces, whose [url=http://www.newfaces.com/model-search.php]search page[/url] is still up and working. I think I've found one playby that way..