The person that originally made the adapter is Hagasan on this forum. I've tried contacting him along with others without much success so my assumption is he's no longer active/interested in producing the part unfortunately. I think you're only option is finding a used one or going the adapter-less route.
Here's a thread I posted a while ago that documents the process:
http://www.nsxprime.com/forum/showthread.php/176307-S2000-Steering-Wheel?highlight=s2000
There's a few places to buy an S2000 wheel. I got mine off craigslist, they seem to come up every few weeks here. The S2000 forums have them on ocasssion along with ebay. The going rate for the wheel with airbag seems to be somewhere around $250-$450 from what I saw. Be sure to checkout Ryu's thread for further details because there are several different versions of the wheel.