I couldn't ever find a brush like this myself, despite seeing people use them plenty of times. Maybe I was looking in the wrong place? Who knows! In the end I just figured out how to make one myself, and it makes sense to share it just incase someone else is looking for something similar.
It won't necessarily be what you're after, and it does take some getting used to, but it's still pretty nice for painting with. Especially when you need hard edges. It works really well on rock I've found, or just for quick, rough painting in general.
Here's a link to the CGHub page that's hosting it, the download link is just below the example image on the right-hand side [link]
As long as you've got a version of photoshop higher than 7, you should be okay.