Thank you Eva Marie and MissC. 
I tried it again very early this morning; a different 36 C with back closure this time but the results were the same. Our local (well only 30 miles away) Sears had a sale last weekend with some bras on very cheap clearance, but I think they must have been pretty well picked over by the time I got there - only one 38 B left which didn't appeal to me and presumably not to anyone else, and certainly no 40 A, but those seem to be pretty rare birds, as do any clothes suitable for a really tall girl - they seem only to grow very wide ones here.
At least I now have a better idea what to look for.
I tried it again very early this morning; a different 36 C with back closure this time but the results were the same. Our local (well only 30 miles away) Sears had a sale last weekend with some bras on very cheap clearance, but I think they must have been pretty well picked over by the time I got there - only one 38 B left which didn't appeal to me and presumably not to anyone else, and certainly no 40 A, but those seem to be pretty rare birds, as do any clothes suitable for a really tall girl - they seem only to grow very wide ones here.
At least I now have a better idea what to look for.