c-span congressional committee coverage and late night celebrity interviews are all good places to find interesting faces.
I pulled these three thumb-nails from a single sketchbook page showing the simple approach that works so well for me. It is very different from having a person pose in front of you in that these people are always moving, gesturing and turning side to side as they converse. I find that quitting any particular view as soon a person changes position or the camera finds another face, then returning to continue a minute later works well. When you do, you then start others that can be picked up, back and forth, in opportune moments. You have to work very quickly in a concentrated way, often so quickly that you can't actually follow the conversation!
Another, and even faster way to approach the problem, is to use soft pencil to catch the darks, the shadows, in as simple a fashion as you can. Forget nuance - pay no attention to detail! It's wonderful how well a few simple marks can capture characteristic expressions or personalities. To me, it's even more amazing that you often get a real sense of three-dimensional form this way. It's great practice!
And, if TV isn't your thing, there's always the cat! Have fun!
"The eye sees only what the mind is prepared to comprehend."
"The freer the form, the more concentration you require."