I have the Tein Street Advance Z coilovers on my 09 IS250 but the spring rates on them are just not stiff enough. Is there somewhere where I can buy stiffer springs that are guaranteed to fit with these coils?

 

You can order some custom swift springs through FIGS(shopfigs.com) I think.

 

BC Racing sells just springs for coilovers

 

Someone was selling some BC springs on lexus isfreaks page a few days ago.

 

Coilover springs are standard, you only need three measurements and you can buy they from a million places. You need to know the inner diameter of the coil, the free length, and the stiffness, then choose something that matches.

Example - the springs in my coilovers are:
60mm (2.37") ID
7" length
12kgf (front), 8kgf (rear)

The manufacturer, or the shop where you bought the coilovers should know those numbers.

Also keep in mind that "stiffer" may not work with the damping in your shocks. Too stiff will make the car ride horrible and the handling unpredictable.

 

Don't go above +2k or below -2k on the spring rate than what your current shocks are valved at. Otherwise you need to get them revalved for the correct spring rates if you do change them.

The Street Advance coilovers have a spring rate of 9kg all around and they are valved for that so upgrading to stiffer springs without going over 11kg would be ideal. You could always change the front to 11kg and keep the rear at 9kg or even put 10kg on the rear.

I have the same Tein Street Advance and I also would like the springs to be stiffer.