Here's a link for a couple of brushes.
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1. Download file to whatever folder you want. (usually goes straight to downloads)
2. Right click folder, and choose extract all files.
3. Open the new folder with the extracted files inside.
4. Copy all of the brushes.
5. Find your Gimp folder, and look for a folder inside of there called brushes.
6. Open your brushes folder, and then paste what you copied earlier into that folder.
7. Go back to GIMP, choose paintbrush, then hit the refresh button in the brush tab.