So to throw in something from my experience - My car that had good springs but needed new front shocks. I had no coil overs on the front though to move over to the Koni's. Called up a few places and with little luck I could not find just two coil overs for a car. Being on a budget for my first season after buying the car I was not looking forward to $300 for a set of four coil overs with springs I didn't need.

Along comes Skunk2 racing. Skunk2 sells a full kit with four coil over sleeves and four springs. They also sell as "replacement" parts coil over sleeves, locking rings, and the Koni shock mount ring. Bought the front coil overs for $100 shipped with the Koni ring adapter and locking rings for the towers. They are nice aluminum pieces and work well.

Note the spacer to mount them is just an o-ring and left the coil over kind of sloppy on the shock when not loaded (in the air). To resolve this we drilled out a piece of aluminum rod and turned down the OD to fit inside the coil over. Not everyone has access to a full size machining lathe but something to think about.