These are 32-bit icons with gradient transparency. The size formats are 64, 48, 32, 16 pixels. If you are new to icon formatting it means that each icon contains four images. The Windows OS will select which size to use based on the view type you're using for a particular folder or the icon size setting in your 'Appearances' menu. Those sizes will look like this on your screen:
There are 218 flags represented here which consist of the 197 current, internationally recognized sovereigns plus various overseas territories, de facto governments, etc. There are many not included. I will make additions upon request. To download an icon, right-click it and chose 'Save link as.' Make sure the file has a ".ico" extention, not ".png."
Click a letter to skip through the list. You can also resize this frame for your convenience.