What is the ruling of the Islamic Sharia on sewing and selling national flags to government bodies, knowing that some consider it an illicit act?

If national flags are intended for distinction and as an emblem for the state, then they are permissible. However, if they are intended to promote nationalism, false tribalism, or contain invalid symbols, then they are not permissible.

Thus, for whoever discerns that sewing these flags falls under the first category, there is no sin upon him. But if they fall under the second category, or are flags of disbelieving countries, then it is not permissible for him to do so.