Familiars is a new feature added in The Sims 4: Realm of Magic game pack, and this allows you to have minions that do your bidding. In order to get a Familiar, you will need to become a Spellcaster. This is done by going to the Realm of Magic and complete a short quest. Once you become a Spellcaster, you will be able to get a Familiar.
So, here's how you get a Familiar once you are a Spellcaster in The Sims 4: Realm of Magic. Go to the main hub in the Realm of Magic; Caster's Alley. There are several different shops in this area, and each shop completely changes their stock every single day. So there is not a single shop that sells Familiars, but all of them. You just need to check the inventory of each shop, and at least one of them will sell Familiars each day.
Their price will depend on their rarity. The common ones (Fairy, Raven, Sixam Owl, and Bunnerfly) cost $100 each, while the Uncommon ones (Dragon, Glowfang, Snub-Nosed Leaf Bat, and Hex Doll) cost $250 each. The rare ones cost $500 for Pheonix and Skull, and $777 for the Velid.
The Familiars they sell are in the form of Familiar Orbs. This item allows you to summon a Familiar to stay with you. You need to bind it to a single Sim first though, and this is done by clicking on the item and giving it a name. This will permanently tie that Familiar to that Sim.
In order to summon your new Familiar, click on your Sim and click on the Magic menu. Now select Summon Familiar, and your new Familiar will spawn right in front of you.
You can have as many Familiars as you want, so make sure to check out the shop every day if you enjoy collecting these. You can however only summon a single Familiar at a time, so the first one will despawn if you choose to summon a new one.
It's also possible to use a spell to bind your already owned cat or dog as a Familiar, granted that you own The Sims 4: Cats & Dogs.