Clock Works- http://www.clockwks.com/
They will refurbish as original or convert to quartz. I have had them do both and very pleased. My current clock was converted to quartz by them about 15 years ago and keeps perfect time.
Corvette Clocks by Roger is another vendor. Roger did the clock for my 64. I kept mine original.
This place repairs/converts clocks AND sells do-it-yourself kits. I bought a quartz conversion kit for my Trans Am and installed it 4 years ago. It was fairly easy to install and still works great. They even sell new original electromechanical movements as well. I had the clock in my Coupe DeVille quartz converted well before I discovered this place. It has been running for 15 years. If it ever dies I'll buy a DIY kit for it too.