I've purchased bulk balsa from different craft sites before. It's SO much cheaper. I just shoot them a quick email first to make sure they aren't treated with anything. I also smell them once I get them to make sure they don't smell chemically.
My birds LOVE balsa so it costs a small fortune to buy the parts from bird supply stores. That looks like it would be a great option (to always have some for back-up) as long as you can confirm they aren't treated.