I'd like to commend a small UK company new to myself, called Maverick Models. They sell flags in different scales for a large number of periods, not just the usual suspects but for some niche periods as well such as Carlist wars, Mahdist rebellion, Texas Revolution and the Pacific war of 1879 to name but a few.
I came across them searching for Swedish flags for the Thirty Years War, and found they did scales from 10mm to 40mm, either plain or 'wavy' effect on plain or self-adhesive paper, or on material.
What I feel makes them a stand-out company is the excellent communication with Stuart the owner, the price of the flags and that the flags are purchased individually instead of a sheet of them, where you end up paying for a bunch that you don't really want.
I found that the 15mm flags I bought a tad small (the figures must have been nearer 18mm) but Stuart was happy to re-scale them to my instructions.
If you are in the market for some flags, I would suggest you have a look at Maverick.
maverick-models.com
worries about not losing his rules stuff.