If you can find pearl barley or just barley kernels, you could put them in your blender and blend them until they are flour. could take a while, but does work.
By the way, I found barley flour from an Amish co-op near our family farm in the Allegheny mountains. If you cannot find it at a health food store, check with local farmers. I also wonder if you could not just grind barley in a blender. Might be worth a try.