If the springs are broken, then you need to replace the springs. Even if they arent broken, replacing the springs is your best bet...
First need to find out if they are "S" springs or a coil type of spring. The "S" springs are ready available from any upholstery supplier, the coil springs are getting harder to find. My guess is they are coils, and if they are sagging, simply get some dense foam scraps, cut into small chunks. Stuff the foam into the coils until it stretches back to the original height. Now the stuffed foam springs will use the density of the foam as the "spring," and your seat should be back to normal.