I used mugshots.com, which has thousands upon thousands.
Nowadays I mostly manage to get the alignment right (well, more or less, anyway), and for such very quick sketches I forgive myself any and all errors if I can manage to more or less capture a likeness. It wasn't always like that. Years ago I had a mighty struggle with aligning eyes. After battling it out for literally hours with a portrait, I decided to sacrifice it to the learning process: I took a ruler and drew a hard, solid, absolutely inerasable line where the eyes should be. Then used it as guide. Only to find that I had misaligned them again, by somehow managing not to stay on the hard guideline. I gave up on art altogether then, but of course, I couldn't stay away from it for long.Badly misaligned the lady on the left's eyes, so now she looks even more criminal...
There's more than one use for rulers.
At one point in my desperation, I started using a grid on a reference photo, thus making sure the shapes were right. My portraits did not resemble the people in the reference photo at all. There is more to it than shapes. What's more, in caricatures the shapes are deliberately distorted, sometimes quite grossly, but in a good caricature the person is nevertheless instantly recognizable.Correct shapes make a portrait recognisable. When I pick my grandchildren up from school I can't see any details of the faces, but I instantly, at a distance recognise the shapes of their head and features. Same with all people we know that we can pick out of a crowd 50 metres away. Get the shapes right and you have a likeness.
Well, that's a good question. Here are two caricatures stolen from the web. Given the distortions of the features and proportions, they should be unrecognizable, yet I have no trouble identifying the two gentlemen:Wonder what it is then?