Opened up the headset on a 1988 Fuji Del Rey, and found that the plastic compression ring was cracked and brittle, ended up destroying it to get it out. Can anyone point me to where I'm able to get a replacement? I'm not sure of what size to look for. Any help is appreciated, thank you!

find out the brand of headset, fuji bought to be installed . bring by LBS for a look.
they can see ..

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.