In a pinch, when nothing can be found, you can often fake a compression ring using an O-ring. You need one of roughly the right ID, but can stretch it a bit. The key is that it has to be thick enough that when pressed into the wedge area it still sticks out above the headset so the spacer doesn't bottom out on the cup itself (or does so at full compression).

Go to the hardward store and buy an assortment of O-rings in the right diameter and have fun with the trial and error.