There are thousands of custom hairstyles you can download in-game, for absolutely free. Most of them come either for free, either with a donation of your choice directly to the creator! But when it comes to hair, I’ve seen custom creations far better than the “vanilla” ones.
The Sims 4 has a bunch of expansions and stuff packs, that besides the in-game features, offer cosmetics, new hairstyles and more.