A question was asked today (titled "Bowed head" reading pose) to find an appropriate image to use as a reference for drawing.
While the nature of the question is not appropriate for Arts & Crafts (or Stack Exchange in general), the topic is very relevant.
In order to get users searching for reference material a running start, it might be helpful to refer them to a thread that collects tips and advice, so please post links to online databases, practical tips on searching specific images online, titles of high quality books on anatomy or that have a vast amount of photos of poses, &c.