They appear to be in very short supply now-a-days! I just googled the kit by FSN and Name and found two on line listings with "Out of Stock" notices and that was it. One for $185 and one for a bit over $300.
I think you would have a better chance of finding one at a military surplus store or MV show.
Thanks for the tip Ron...Got the complete adapter set from that guy on G503 for 75$ and also found the dwell/tach kit with instructions for 100$ on Ebay