You actually buy fabric for foundation piecing?
I have a stack of fabric that I have labeled "trash fabric". That is what I use. I needs to be very light weight and doesn't need to be new. The best thing to use is the top part of an old fitted bed sheet. The sides are probably still quite sturdy, but the top gets worn out first. I use anything that doesn't have an actual hole in it. And even a few small holes don't matter, as it is covered in whatever you are sewing to it.