[Edited]
January 2020 - last edited January 2020
@Sophiexxxxxxxx, a little bit of clarification. If you just want hairstyles, clothing, furniture, eye colors, etc. then what you want is called Cutsom Content (CC). But if what you want changes the actual gameplay than that is a Mod.
But, if you get Mods or CC you will need to maintain them. That requires a bit of work on your part.
Every time the game is updated by EA, like right before a pack is to be released, there is a chance that some Mods/CC can become outdated. When Mods/CC become outdated, you need to replace them with updated ones, if the maker has updated that item. If you don't replace or get rid of outdated CC/Mods, then your game will have problems. 
You can always check for Mod/CC conflicts and clean up any corrupted data from them by doing the following:
1) Move the Mods Folder to your desktop
2) Delete localthumbcache.package file in the game folder.
3) Fix you game in Origin by logging in and go to Library, then right-click on The Sims 4 and choose the "Repair Game" option from the list. Wait until Origin finishes repairing all your packs and not just the base game.
4) Now, when you turn on your game again, it will create a new mods folder that will be empty. Check to see if your game is now working without mods/CC.
5) Now you can try adding back items from your Mods folder one at a time checking your game each time to see which item is causing your game to not work properly. If you have a lot of items, you can try the 50/50 Method.
Then you can go back to the CC/Mod creators and get the updated versions when they are available. 
If you still want to use Mods/CC, then the sites that @Psychotps gave you are good ones as they have been around since at least The Sims 2! (I used to use Mods and CC in TS2 and TS3, but no longer do so because I don't want to have to keep up with maintaining them.
) But please keep in mind everything that @Psychotps and@RandomBuzziness suggested about downloading.
Happy Simming! 
