Front leaf springs

[Motor City GMC]

I have a 1987 GMC 1500 sierra 4x4. I need to replace the front leaf springs, due to the ones that are on it are bent flat (straight) if not bent slightly downward instead of upward (if i am wrong please let me know)I just want to put stock back on but I know nothing about leaf springs so how do i know if these are stock or aftermarket and how do i measure them to get the ride height to match the rear. The truck sits a little high from the factory so could there be a lift on it already? How do i tell? Please help and if anyone knows where to purchase them from for a reasonable price. Thanks to all!!