If you do not use the decal sheets for flags, here is a trick that some people use. A google image search can find many flags. An image editor can easily resize them. And, many editors provide a mirror function. You can copy a flag and mirror the copy. You then can move it next to the edge of the original image. When printed you fold at the common edge and end up with a two-sided printout. And you can put a piece of thread in the folded edge to allow you to attach the flag to the model.
I even know folks who tape a piece of thin tissue to a sheet of ordinary print paper to end up with a tissue flag that is easier to wrinkle after folding and gluing. Glue sides together with thinned white glue.