Havard's website may have it. Also you could look in your graphics folder - the codes are included in the flag and shield filenames.
If you're doing modifying, be aware there are a few not used there (SLZ, Salzburg, is an example), and there's also 20 that were coded in (U00 to U19) for user use and do not belong to any country specifically. You cannot create new country tags, other than these 20 and the ones (around 150-200 I think) that already exist. A couple (FRA, ENG, SPA, POR, PAP, KNI) have special hardcoded properties and probably should not be reused unless you're aware of those.
They're generally the first 3 letters of the country name, but there are some cases in which this does not match. (HAB, for Austria, comes to mind)