ISO Codes

Example - ''Use a nations ISO code to summon the corresponding flag''.

Question - Where/How do I find ISO codes?

Any help appreciated, thanks.

not sure if it will work but found the codes here

click on the letters for the nations

It's the 2 letters official ISO codes of the nations.
You can find them on this Wikipedia article.
The Alpha-2 codes is what you're after

Examples:
gr no nl es fr

Sure, but it doesn't help much if you're after something that doesn't have an ISO nation code. Like Scotland, which clearly has a flag, a football association and league system of long standing, but is not considered a separate nation in the sense of this coding system since it's part of the UK.