If you're looking for laun plywood,a building supply place sometimes has damaged interior doors and sells them cheap.I got some birch doors locally for $3 a door with some beautiful grain and color.If you're looking for aircraft grade birch, Aircraft Spruce
www.aircraftspruce.com and National Balsa
www.nationalbalsa.com are good places.Aircraft grade is the better of the two with it's glue being waterproof but laun can be used if sealed inside and outside really good.The laun doesn't like harsh bending and will split if bent too far.