I have used almond as well as hazelnut meal instead of flax meal, replaced some of the spelt flour with teff flour, used banana instead of sweet potato, etc.. (scandifoodie.blogspot.ru)
I ran out of almond flour and used teff flour for the remainder 1/2 cup I needed, I was determined to have coffee cake this morning! (elanaspantry.com)
You'll need to either grind yours if you have a high speed blender or a grain mill, or look for teff flour in a health food store (I believe that Bob's Red Mill makes one as well). (golubkakitchen.com)