You can find various Custom Content on different Sims fansites, ModTheSims and TheSimsResource are a couple of sites that offer them, as well as some independent artists/designers that have made their works available on their own art sites. I recommend Googling for Sims 4 Mods with what you're looking for; most custom clothing is pretty easy to find this way.
As referenced earlier, you will need to put the Custom Content files into your Mods folder, which can be found in the following documents folder on your computer:
(C:\Users\UserName\Documents\Electronic Arts\The Sims 4\Mods)
You can create SubFolders in the Mods folder, but I'd only recommend creating single-level folders to keep your content organized. Creating folders for "CAS" or "Scripts" might be easiest.
Finally, to activate Custom Content in-game, you must do the following:
- Open the Game
- Go to Game Options > Other
- Make sure "Enable Custom Content and Mods" and "Script Mods Allowed" are enabled. (You will need to restart the game thereafter so the game will load the mods.)
Once reloaded, start the game, and check the areas of the game where your custom content should be visible; they should now be available.
(Note: Whenever the game has a patch, Custom Content will be temporarily disabled, which the game will notify you of. You must repeat the steps above with opening the game and turning the custom content back on to allow them again. Be mindful that some mods fall out of compatibility with the game, especially scripts; make sure you keep track where you get your mods, so as to update them if they no longer become compatible.)