Here is a nice collection of country themed mail stamps. You can change your default mail.app icon to match your country. There are 31 countries included and they look great.
These images are delivered in .png files, so if you want to use them as an icon, you will have to use a program like img2icns. It’s real easy and has a great finished product.
(If you enjoy customizing your icons and you are a fan of NBC’s The Office, you’ll enjoy the icons posted in this post on LifeInTheOffice.com. This also has general instructions on how to change your icons.)