Stock photo sites are usually excellent, especially if you're still figuring out how you want a pose to go.
The best is a family member or close friend whose build/personality is similar to the character you want to draw. Photos of yourself are almost as good but camera angles get tricky.
Last up is a 3D pose program (I like DAZ) but it will give you poor results if you aren't already proficient with pose fundamentals and 3D rendering, or if over-relied upon.