Have you done a google search yet?
I came up with 784 000 results when I search "photoshop brushes" That should be enough to get you started i reckon. ;-)
I'm not gonna post any links coz I haven't used any other brushes really except those that come with PS so I can't recommend any site in particular.
Remember - Google is your friend.... also make sure you run a virus checker over anything you download.