Familiars are not physical creatures, they are spirits that Aid a Witch in their practice, generally they appear in animal 'form' which is part of what lead to the legends of Witches having a pet that would fly around the town at night, harming innocent villagers and drinking blood to feed a Witch and all kinds of fun things.
Familiars can be 'obtained' in a variety of ways - I may make a post on this in the future.
Coven Given - as in your Coven or Gathering would give you access too a group familiar or would help you obtain one when you're ready.
Born With - This more falls under Fetches, which is another kind of personal spirit of a Witch, essentially the same thing with a few distinct differences. Essentially when you're born, the Familiar is born with you.
'Summoned' - you can reach out to the Otherside for a spirit to form a working relationship with.
They find you - They will turn up, almost out of the blue and pretty much go 'I want to work with you'.
Just because you want a Familiar doesn't mean you'll ever get one, some Witches never have a familiar in their life, some have more than one, it's just how it is.