i have an older home the windows are counter balance with Pocket weights. I ma rebuilding the windows and I want to insulate the pockets. I have seen a product that replaces the weights and pulleys with a spring loaded sash lift. The mechanism fits into the slot where the pulley is located. A flat strap replaces the cord and a spring( a larger watch maker type???) replaces the counter balance weight.

Any idea where to obtain???

Thanks