It is quite hard to find good free photoshop brushes. This was the first website appearing for the keyword and it is a great place for everyone to share and download photoshop brushes. More importantly, most of it does support quite a high resolution.
I believe there is close to 500 brushes and it comes with a great search and filter function to look for the brush you need.
Here are two other sites that I would find my photoshop brushes.
Leave me some comments if you have any brushes source to share.