LMC has those springs. Here's a link, Front Bench Seat Components Item #3
http://www.lmctruck.com/icatalog/cc/full.aspx?Page=72
They fasten to the last spring, the one that goes from front to back of the frame. Use these 3 prong fasteners, 2 per spring. So if you use 2 springs for each end, that's 4 springs and 8 fasteners.
http://www.ebay.com/itm/321277999753?var=510200784460