There's loads of usable stuff here on DA. I've been using this person's stuff a lot lately. [link]
For life drawing purposes, avoid fashion shots like those posted above. You don't want the face to be obscured or bleached out.
Or you could always sit in front of a mirror and draw yourself. I'd actually suggest that more since this is a life drawing class. Drawing from a photo and from life are quite different skills.
Try `mjranum-stock - super high resolution with gorgeous lighting and isolation. They are full nudes, but the quality is enough on most of them that you can just crop into the portrait and have more than enough detail for a drawing/painting.
Problem with that is, I don't have anyone willing to let me draw them, and most of the stocks i've been finding are meant for more photoshopping.
I was wondering if anyone had any suggestions for Stock images, shoulders up, with strong lighting/shadows?
I appreciate the help ahead of time.
meanwhile i'll keep looking...