Why not making yourself? It's much funnier and also for rookies doable.
Short explanation in steps:
1) With AoE3Ed's ArchiveViewer extract from "art2.bar" the files "Art\objects\flags\[civ].ddt" and "Art\objects\flags\[civ]_32.ddt" to a temporary directory.
2) Use AoE3Ed's FileConverter to transform both to .tga format.
3) Use a graphics processing software (capable to .tga) if you want to draw yourself or simply a good viewer (eg. IrfanView) for picture exchange and sizing.
Important: keeping the original gfx size and filename.
4) With FileConverter transform the edited files back to .ddt and place files in "[program directory]\Art\objects\flags\" (in case create).

Sorry to take so long responding. Where can I find ArchiveViewer and FileConverter? And can I directly convert a .bmp, .gif, or .jpg file to a flag?

Using forum search could have brought up this article

Yeah sorry Patine, haven't visited this forum lately. Absolutely no time for anything these days so I will have to pass. Best advise I can give, had a read of the flag tutorial I wrote, it explains everything.