You can find pretty much any flag on Wikimedia in .svg form, just scale it to your desired resolution in an image editor or simply set the resolution in the browser and download it.https://upload.wikimedia.org/wikipedia/en/thumb/a/a4/Flag_of_the_United_States.svg
Here is the current US flag. Search "List of flags of the United States" for other historical or alternative flags.
Add "/0000px-Flag_of_the_United_States.svg.png" after the URL with 0000 replaced with your desired resolution to get a downloadable preview.
SVGs are truly wonderful, aren't they?
Here is a 2850x1500 png. Less than 50KB? PNG compression is great!