There are several good websites out there, but the one I've found to be the best (and probably the largest) is Mod The Sims (
link). Not only are there lots of new clothes to download, but plenty of other things too, like accessories, houses, careers, furniture and even new sims (including real-life celebs). There's also a good forum there with a lot of very experienced people who can help with just about every problem you may have with the game.
