Tradition also states that familiars are received at initiation into a coven from a Magister or granted upon the witch by spiritual entities (Faust is a good example of the use of a pact in order to get familiars)
Another way I've found quite interesting, is while in a section of the astral (inner) plane by the use of a symbol (a tattva or a tarot card for more experienced practitioners) is meeting a certain individual and requesting it's help, also it's sigil and name, so it can be called upon need.
Another technique, powerful, but disastrous if it goes wrong, is creating an artificial elemental being, also called servitor by chaos magicians, or artificial familiars. You picture it's shape, charge it with force by appropiate invocations and give it a task to fullfil. Now, always, after the task is done, or if it fails after a period of time, destroy, banish, etc the being...if you don't, then it will become contaminated and wreck havoc in you life...also, it will be harder to banish.....