Get a pocket-sized digital scale. If you have a smartphone, the NCLRA website has a balsa density calculator on it. If not, make up a chart of minimum acceptable weights of balsa in commercially available sizes. Then, once you have some time to kill, go hit places like hobby lobby, hobby town, michaels, and anywhere else that sells balsa. A diligent worker can sort through a shelf of balsa in about 20 minutes or so. Revisit these places on about a 1-2 month basis and build up your supply. I am building an Arthur Alfieri "Ruby" and every piece of wood from 1/4" down is wood I picked up from Hobby Lobby and Ace Hardware, and it is all within the 5-7 lbs range. A little time will save you shipping costs, plus you get to drive home knowing for sure you got what you needed.