I found an easy solution for finding a disc for the switch.
Just go to the hardware store (with your tailcap in hand) and look for a washer that will fit. You likely won't find one that is a perfect fit...
The one I found would go into the tailcap but was too large to get past the internal threading. I just hand sanded it and now it fits perfectly.
I found a spring that fit perfectly inside from an old flashlight and soldered it to the center. Once I'm ready for the final assembly, I'll glue the washer into the tailcap.