One trick I found useful in buying light balsa is to ask your local hobby store to call you when they get a new load of balsa in. I lucked out when I was buying wood as they had just received a large order. I took my digital scales in and weighed each sheet. For 1/16" 36" x 4" sheets, I only accepted balsa that weighed 12 or 13 grams. (Need to have a good relationship with your LHS!!!)

Steve