I split your post into a topic of its own so we won't be hijacking someone else's topic. :)
Your question doesn't have a straight forward answer. It really depends on what your resources allow. My number one suggestion is to find stock photography that allows you to use their photography to create drawings with that you can sell. (This is very important, not all stock photography artists allow this, so always double check with their rules!) For some you may need to purchase a license, others give you free use of their stock provided you credit them etc. It depends on the individual photographer on what they allow, which can really limit your material.
The same applies to models, though you might end up in a much bigger gray area than I can cover. Models have contracts, photographers usually have contracts, and these contracts determine how the material you'll create can be used. You can have a model waive all her rights to the image and only go home with the pay you give her. A model can insist on cashing on on royalties from the derived sold material etc. This really depends on what's agreed upon up front, and may require a lawyer to cover your bases.
When money is involved, things get trickier than if you were just a hobbyist with no intend to sell. I highly recommend you research the legal aspects on these things before you get in a situation that overwhelms you because a model you photographed suddenly changes her mind. I hope this helps you out!
Just to confirm with you, the references on this site are not to be used in artwork you intend to sell. We have a bunch of different licenses, some photographers allow selling of the derived works, others do not, so your safest bet is to not sell the artwork you create with our references. I'd hate to see you get in trouble! We will overhaul our reference library at some point and the licenses will for using and selling derived works will be looked over, so keep your eyes peeled on how that works out. :)