To answer your question:
There are probably more "grunge" brushes available than all the other kinds of brushes put together. Have you looked at the Resources section here? That's one place to start. There are also several threads with links to brush sites (find them by using the Site Search tool at the top right of the screen), and then of course there's also The Search Engine Of Your Choice - type in "photoshop grunge brushes" and there you are.
__________________ "I might join your century, but only as a doubtful guest"
There are awesome grunge brushes here. But I've also found some really good links to stuff like that by searching about.com.
Hope you find what you're looking for!
DeviantArt also has some really great artists and brushes for download, personally I would suggest searching for JScully, her brushes rock and as always, as my mother always said... "google it! "
and as always, as my mother always said... "google it! 
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