Detroit Springs. Moog, and Eaton all sell replacement coil springs for your car.
YOU have to decide upon ride height (free height and spring rate affect this), and how you want the car to handle/ride. You can not have a good handling car with soft springs, nor can you have a smooth ride with stiff springs. If you are interested in handling (turning a corner without rolling over) you also have to consider shock valving and sway bar size.
You need to know your car's actual weight with you sitting in the drivers seat. Four corner scales work best but driving half onto a scale at your truck stop can give you front to rear weight bias. The center of gravity of your car is located at the top of the engine in the middle of your dash (X and Z axis).