On my paper model website I offer various Flag sheet sets.
Printable flags that can look quite realistic when done.
Many ship builders have used them.
My US Naval Flags sheet covers all Navy vessels throughout WW2 up to 1959.
There is a second sheet that has all the radio signals and various pennants, and state flags.
Anyway, the flag sheets are at the bottom of this page:
https://davesdesigns.ca/dcc/html/free_models.html
Just click the one you want, a PDF download will happen.
Print the flags on on lightweight paper (the thinner the better).
If the size is incorrect, you need to decide how much bigger or smaller you need them and reprint.
You can adjust the scale up or down by percentage during printing.
Carefully cut them out with knife and/or scissors.
Most are folded down the center line, over a "mast", and then glued back to back.
After the glue dries you can give them a bendy wave.
Don't bend them too much because the artwork already shows a wavy flag.