For an airbrush:
1. Go to the plugin section and download Sabrown100's Custom Brush plugin. (It's stickied)
2. Find the "Share Your Custom Brushes here" and look for usedHONDA's post on the first page. After running the Brush plugin once, download usedHONDA's brush and extract it to My Documents\Paint.NET User Files\Custom Brushes. Restart Paint.NET, run the plugin, and on the left pane, there should be an airbrush. Enjoi.