RE: source for 60" balsa sheet?
Remember, that you had better want a lot of sheets. The 60 inch size is not too popular right now. I am having 1-2 month delays in obtaining 48 inch long stock as well. I would expect that you may be looking at a 3-8 month delay.
Of concern to me is that shipping will be rather expensive. UPS will be only domestic place that will accept shipping parcel.
Wm.