Which rear spring to buy

[powerpocok]

Hi, I have a 2007 1.9 cdti 150hp station wagon with standard suspension. The car is barely moving when I try to put body weight on it, the suspension is very stiff. I have oem springs with constant wire diameter.
Is there any brand that makes softer springs?
What if I put spring with inconstant wire diameter, it will work?

[bibu16v]

Theoretically it'll work. As concept, springs with inconstant wire diameter become stiffer as load is increasing. In the beginning they should be softer. But it does matter too the 2 wire diameters to compare.