This may me a little costy. But I'm pretty sure you can do this: Try sending a weather balloon up to the highest altitude you can (do it higher than 40,000 feet) and stream video feed from a camera.
They've got this one covered actually.
https://wiki.tfes.org/High_Altitude_PhotographsAlmost commendable. It's another "Heads we win, tails you lose" argument from the Flat Earthers
FE: "Look out your window. It looks flat. That's evidence of a flat earth"
RE: "That is just the scale of the earth, you can't perceive the curve at ground level but if you get high enough you can. Look at this photo/video"
FE: "That's from a space agency. FAKE!"
RE: "OK. This one is not fake, this is from some hobbyist who sent up a weather balloon and you can see from the images on the ground there is no lens distortion"
FE: "Curvature results from the fact that at the edge of the atmosphere we are looking down at the illuminated circular area of the sun's light."
RE: "What? You literally just said that the first image was fake! Now you're saying you DO expect to see a curve?"
It's quite hard to argue with people who think like that. Some people sign up here and are genuinely interested in the arguments. Those people probably can be persuaded with logic and reason that FE theory is bunk. To answer your original question though, others are so entrenched that I don't think it matters what you throw at them, they will always find a way to deflect or deny or ignore the things which prove them wrong. If you understand a bit about confirmation bias and cognitive dissonance then you'll see it at work a lot on here.