I have 6 over 6 and 6 over 8 double hung wooden windows in my home. 
The largest of the windows (upper and lower sashes portion only, not
including the frame) measure about 3 feet wide by 5 and a half feet
high. The spring balances need to be replaced on almost all of
the windows. (The sashes will not stay up.) The house was built in
1986.  I have tried to obtain it but have no information as to who
manufactured the windows. The staff of the wholesaler who provided the
windows to the developer in 1986 says they have no records of who the
manufacturer was. (This seems odd to me.) The local lumber yards are
not interested in finding the spring balances. Where can I find the
proper spring balances for my windows?

Hi,

They may use constant force springs.  Also known as Flat springs. They
provide a constant linear force over a long distance.  MSC has them.
Go here:

http://www.mscdirect.com/PDF.process?pdf=1240

All the best,
James