These are what i referenced:
Swift Race Spring Thrust Washers | SPL Parts
They're pretty cheap, and for the rear, due to the springs being separated from the shock, the install is super easy. Just get the car up, drop the arm, u can stick it in between the spring and mounting spots, and you're good to go.
on coilovers you'd need to disassemble to slip them in
solved all groaning/creaking sounds on the back of my Z after coilovers, i used them on my custom ground control/koni setup on my S14, and i installed them on my Fortune Autos for my IS300
no more noise from spring v perch on compression, its a great solution
there are thrust BEARINGS that are more complicated, expensive, and require maintenance