This and this. Its not so much who they are from, but what they say and how credible they are. I had a letter from a previous supervisor who an AD crew chief, a previous supervisor who happened to be a retired colonel in the marines, an MTL from tech school, and my current shop supervisor at my ANG unit.
Get as many as you can, and pick the ones that you feel speak the best about you. Don't submit a letter just because it is from someone you think will impress the board.