..what about buckwheat? The color and taste are about as close to teff as I can think of.....

Syronai is right on. I have been fortunate enough to spend a lot of time in Ethiopia, home of teff, and teff means 'lost'. Becuase the grains are so small if you drop them you can't find them any more.

Are you using the flour or the grain? I think SKK's right in that millet would be a good substitute, though the grains of millet are significantly larger than those of teff. If you're using the whole grain, you might also want to try sesame seeds or chia seeds. If you're using flour, pretty much any gluten-free finely ground flour will probably work. Make sure the grind is very fine though; teff flour is so fine it practically melts in your mouth, so you'll want to try to mimic that texture.

Don't know the recipe, and think substituting millet would work very well.