For those who still partake in building I've listed a box of balsa sheets on eBay, all new.
(36"x4"x1/8) 86 pieces
(36"x4"x3/32")46 pieces
(36"x4"x1/2") 4 pieces
(48"x4"×1/8") 6pieces
Plus loads of triangle
https://rover.ebay.com/rover/0/0/0?mpre ... 3270719462
It's all new and stored in a cardboard box in my dry garage.