I can't speak for everyone, but I usually just crop the image that I'm using for the main body so that it zeros in on the face. Lazy, I know, but it gets the job done.

renders? lineart? or commission for someone? you don't make yourself very clear.

If you mean you are looking for blank head linearts to colour for your characters' head icon? Well, to be honest that seems pretty straight forward. The best places to find blank linearts of head shots are on Deviant Art, or in members' galleries on Balto Source who you know make blank renders and linearts for the public to use.

I'm assuming that's what you mean.