Kallman, if you mean, what material do you apply the transfer to, I suppose you could make the flag with paper. I know some do and get good results painting onto the paper, but I'm not sure how paper would work for a waterslide transfer (I would worry about the paper absorbing the water too fast and the transfer sticking in a spot I don't want it). Maybe if you sealed the paper flag with varnish before applying the transfer it would work well.
For my own flags I have had success using the lead(tin) foil from around the tops of wine bottles. That foil used to be on every bottle, but now is only on the more expensive ones. Anyway, it is thick enough to hold its shape, soft enough to cut and roll onto a flagpole and I find it takes paint reasonably well. When I use a knife to cut the foil off the bottle, rather than tearing it, I can usually get a usable sheet about 30mm by 50mm.
Alternatively, some art supply stores sell empty oil paint tubes for artists who make their own pigment/media mixes (basically like a toothpaste tube, but empty). The ones I've seen are made of very thin brass sheet and if you cut off the end with the cap and open them up you get a sheet about 80mm by 100 mm. It's a bit harder to cut than the wine bottle foil but sturdy scissors will do.
You might also be able to get similar thickness brass sheet at a hobby store that carries model railroad or scale model detailing products.
I hope whichever way you go it works well!