Stiffer springs are generally only available for our cars, that will also lower your ride height. You need suspension travel and actually need more than our cars are designed to give.
Unless you find an airbag setup that will fit on your stock rear shocks, to maintain height when loaded, you are not going to find a "towing" spring for our cars, directly under the application of Colt/Summit/Mirage.
You can contact companies like BG, H&R, Eibach, Swift, King, and Whiteline and see if they have any springs that will match your overall diameter, top and lower spring fitment clocking/round-shaved coils, to fit your rear shocks. Then you will have the bigger issue of shocks that will blow out in short order due to the springs being too stiffly sprung, and the shocks being unable to control the stronger oscillations. You'll be chasing your tail at this point.
If you are overloading your car, or the roads have crazy potholes/surface variation, this is not the car to be driving and looking for truck like ability or newer car ride. Your only bet is coilover/external reservoir rally shocks. They are worth more than your car, twice over.
Bit of a dilemma as you can see.
