Control-click the Mail program icon in the Finder. Choose Show Package Contents. Navigate to Contents/Resources/.
The basic app icon is app.icns. *.icns files are standard Mac OS X icon files. I recommend using Iconographer for handling this format.
The badges that are overlayed for new messages are newMailBadge1&2.tiff, newMailBadge3.tiff, newMailBadge4.tiff, and newMailBadge5.tiff. Any basic graphic app that handles tiffs (such as GraphicConverter) will work for these.