Flags are traditionally made from ordinary woven fabrics. Search for examples of linen or canvas, those will turn up examples of the kind of stuff usually seen in outdoor flags.
For indoor flags, silk or nylon are pretty common too.
Could you find or buy a length of fabric and photograph it (or even run it through a scanner) to make your own? That would take care of the royalty problem.
Could you find or buy a length of fabric and photograph it (or even run it through a scanner) to make your own? That would take care of the royalty problem.