I was pleased to learn that Balsa is a renewable resource (unlike cork, which is becoming scarce). The plant grows like a weed in tropical climates and in only a few years a log can be harvested. What I have read describes the plant as more like a weed than a tree, with a pithy interior which is the balsa.
You may be able to get balsa "sticks" at your local hobby or even hardware store. It is always good to buy stuff like this in quantity, especially if you are going to order by mail. I find the 1/2 by 1/2 extremely useful, but it may be worth stocking both smaller and larger sizes.
Pay attention to both minimum order quantities and shipping charges. These outfits like to sell in 48 inch lengths. I like Balsa USA because they have no minimum order. National Balsa does have a $25 minimum order, but this may ultimately not be an issue since it wouldn't make sense to pay shipping on smaller quantities.Tom's Mineralogy Info / tom@mmto.org