Cybertenko has this Captain;s Uniform for M4: https://www.renderosity.com/marketplace/products/127416/universal-sailor-the-captain

Along the same line, https://www.renderosity.com/marketplace/products/72794/dress-uniform-m4 may be useful for making said uniform by tweaking textures.

And there is an actual, quite old flight attendant uniform: https://www.renderosity.com/marketplace/products/65250/flight-attendant-uniform-for-v4a4

Predatron & Diane's General Uniform and add-on for M4 on Daz store. It's a lot more military than airline, though. I guess no one has thought about airline uniforms as most of the pilot suits are still futuristic sci-fi military affairs.