Need help finding the correct spring

It looks like you need a new pivot shoe, too. Please see our 15-231. I think this matches what you have quite well, but see if you agree.

As for the balance spring, I can't make out what you have from these photos. Can you remove an undamaged version and show me a clear picture of the top end, bottom end, and the clip at the end of the cord? That's the same clip that goes into the shoe. Finally, please give me the exact weight of the sash, down to the ounce.

Thank you very much. Your pivot shoe looks like a match for our 15-231, though I don't think your exact style balance is made anymore. The good news is that you should be able to use our S770-27 with a 27C stamp to replace it. Look it over and let me know if you agree. 

Finally, concerning that lock, I'm afraid we don't even have a substitute for this. It's just not a part we can help with at this time. I apologize for the inconvenience. 

You definitely can't use spreaders on an aluminum window, no, but you can make a cutout. In fact we have a video showing you how to do just that. You can see it below.

And believe me, I know exactly how you feel. Some window manufacturers make their windows without any way to repair or replace little parts like this and it can be infuriating. I can only assume that they would rather you buy a whole new window from them than a little replacement part from someone else. Fortunately, we have a way around that. I hope this video helps.