It is not always as difficult as it seems.
As a child I drew many portraits from magazines. We could not afford a variety of periodicals, so my selection of images was limited. HoweverI made dozens of small sketches and I've learned a lot doing so. Now times have changed. If you want to study portrait drawing and models or magazines aren´t available, then do this: Search in Google for "faces". The section images gives you a choice of thousands of faces. People with fair skin, dark skin, young, old, happy, angry etc. Never it was so easy to practise your drawing skills. So, no excuses anymore! Sit down behind the screen (put on your favorite music) and start drawing. Have fun!
See also this post.