If you know what spring rates you want you can buy springs in a few different rates, or you can get springs made to your specs.....
Or Tein, whiteline etc offer the package type deal where they have done a bit of research and put together what they consider to be a good combination.
Also dont forget other suspension mods which if done well you can see a vast improvement in handling too, sway bars, adjustable camber etc......
Do some research before you outlay the $$'s, you generally get what you pay for though.
Cheers,
Russ