The "pros" (and anybody else lucky enough) use live models. Most stuff that you'll find in magazines and online are not hires enough or contain "printing artifacts" that will interfere with the quality of texture. To compound the problem, most photos will not be optimal for transfering as textures - highlights, shadows, clothes, hair, facial expressions (you want neutral), odd angles, occluded features, etc. If you are not looking for hires textures, you might get something good from very high-quality color prints (books, mags, posters) that are large (8.5x11 min.). You'll have to either make the texture small enough to remove the printing artifacts as subpixel or use a tool to remove them. Some image scanning software does a good job of removing the "print masking".