Figure drawing classes in college, biology classes including Comparative Chordate Anatomy, independently purchased anatomy books, as well as books of candid street photography I find at book sales to get people in more natural poses.
I like using the 3D posing dolls available in Manga Studio too.
Finally, I frequently take lots of photos of myself, friends and family members, as well as using online photos for reference. For those I use public domain and royalty-free pictures and deviantART stock that carries no limitations. No sense borrowing trouble.