Wow. Didn't realize they were that hard to find. I got one with my '72 and absolutely love it! It's very sexy Imo.
I found this guy on eBay: http://myworld.ebay.com/ebaymotors/teakhouse/
It looks like he makes custom shift knobs. From what I understand you would just need to tell him what size threads you need. Looks like pricing would be between $15 and $20.
