Tom, I'm surprised that you're not finding suitable replacements; most hardware stores have drawers full of various size & type springs. They may not have enough on hand to do all your windows at once, but usually can get more for you in a short time.
Are you looking for just the spring itself, or the springs as part of an assembly?
Another place to look for springs that might work would be an automotive supply; they, too, often have a large selection.
If these don't prove fruitful for you, I think it would be possible to extend the life of them a while by shimming the springs with washers, or perhaps rubber faucet seats. They would need to be the same size diameter of the spring containers, and could be put in at one end of each spring to take up some slack and give the spring a little more pressure. However, this would only work for so long, as the springs are already weakened, so I'd keep hunting for new springs; certainly don't want to have to remove your window sashes more than once if you can avoid it.
If none of this helps, post back with the dimensions of the springs, and we'll see about hunting some up for you online.