We have been using 6" X 1/4" Poplar from Lowe's quite a bit for 2D & 3D laser engraving with very good results. We try to select boards that are not real grainy, without knots or has any green tint to them.
We just purchased some 12" X 12" X 1/8" Baltic Birch Ply from Amazon recently that engraves fairly well. http://www.amazon.com/gp/product/B00GQN ... ge_o05_s00
We have done allot of mirrors in past, but it's hit and miss finding any with an even white coating on the back of them. The grey backed ones do not engrave very good at all.
White Artist Canvas gives some very good grayscale results, but it's tricky getting the settings just right with it.