A pair of V-Maxx 100-20 secondary springs and spring locators. That's good for one end of the car. Usually you would only need this kit if you're changing from a single spring set of Sport springs and going to Track springs that require the secondary spring.
100-20 springs are 100mm (4") long and have a 20 kg/cm (112 lb/in) spring rate. They are for 60mm ID springs so they will not work with 2.25" or 2.5" springs.