Paul i agree with John! i have 3 or 4 modeling shops quite close to me and every one of them stocks balsa. A bit of advice though if you dont mind, dont forget to buy a tin of dope and give the balsa at least 3 coats as this will harden the finished floats! (i have made my own floats for a while now and believe me its a right pain when you have taken time to make them and the first cast hits a tree and you reel in to find a big dent).
