Use Lonestar, National, or Balsa USA to lay in a stockpile.
Since you said you have a Michael's store- watch the Sunday paper for their Ad. They usually have a 40-50% off coupon good for 1 item. If I can get it from my wife I will take a few minutes and go through the balsa bin- It's Midwest balsa, and ranges from rock hard / heavy to pulpy/ useless, but there is usually some fair to decent pieces. I tend to get the 3/16" or 1/4" sheets.
1 piece at a time, of hand selected decent ( not contest grade) wood adds up, and you also get to look around at the other craft stuff for things that may be useful.
The real deal is if you also launch model rockets- they stock a pack of 24 Estes engines- from A's to C's that usually goes for about $50, getting that for 1/2 price and spending an afternoon with some kids launching rockets is great!