Here are some suggestions:
eBay
Recycled Cycles
Sellwood Cycle Repair
Harris Cyclery
These all specialized in used/hard to find parts. Good luck.
One other thing...are you certain that it's a 23mm you need? My 979 takes a 25mm, so I was just wondering. If you need a 25, I have a Shimano post that I'll give you. Let me know.