Most commonly the sending unit is defective. Some potential clues include the light flickers rapidly, and the sending unit is leaking oil. They don't always leak when they fail but if it is it will have to be replaced anyway and that will likely solve both problems.

Are there any leaf springs that can be retro-fitted for my 64 fairlane ive got one thats broken and all the ones ive found online are pretty expensive

I would look for a suspension and alignment specialty shop. We have one in town that re-arcs springs, and they can build them with as many leaves as desired for any application. They go by length, width, and spring rate. Other than that, they don't care what car it's going on; cost is the same regardless.