Magic Brooms are a new type of item added in The Sims 4: Realm of Magic. The Brooms are used when casting certain type of spells, and they are optional, do you don't have to buy one unless you want to. However, you do get some cool animations when you use the Broom for a spell.
Anyway, to get a Magic Broom, head to the Realm of Magic. If you haven't been there before, read this guide about getting to the Magic Realm for the first time. Anyway, once you are there, go to Caster's Alley, the store area in the Realm of Magic. There are many shops here, but they change inventory every day, so you never know exactly what they tell. However, at least one of them sells a Magic Broom, so just keep checking the inventory stock of all the stores until you find one that sells a Magic Broom.
After your purchase the Magic Broom, go to your inventory to use it. In order to bind it to your spellcasting, choose to set it as a favorite, and you will automatically use that broom the next time you cast a spell that involves a broom.