The problem we had with most flag icon sets is that they were overly designed or unevenly high (11px).
To solve this problem we started our own and decided to open source it via GitHub. We're starting off with Europe but many more will follow.
You can grab them here: https://github.com/hiddevdploeg/SEOshop-Flags
They're all 16x12 / 32x24 and it includes the .Sketch file for styling as well.