Well actually, the problem with using emoji for national flags is this: [ https://en.wikipedia.org/wiki/Regional_Indicator_Symbol ]. The short of it is that rendering country code emoji as flags instead of letter codes is optional, so it will always be hit-or-miss as the OS and/or application must support it, either with the application completely supporting everything independently or by the app partially depending upon the OS.
Now if you wanted a Free flag icon set, I've attached the flag set from Teeworlds. It is licensed under the Zlib license and takes up about 1.1 MiB.