Contact the staff at
http://www.thenismoshop.com/ It's ran by West Covina Nissan, and last I dealt with them, they can order of the Japanese parts catalogue. That's the more expensive route as ordering from Nissan will cost a considerable amount more. Your other options are to watch ebay, forum classifieds, and upgarage. There are no other alternatives to S15 outputs as far as I know. You can also try companies that specialize in parts importing. That can be a bit of a crap shoot as many of them just have dibs on a grab bag of parts as they come into customs ports.
One thing to be careful about is that the left and right outputs are different lengths. So if you find someone selling singles, make sure you don't buy two of the same length.
JSpec has been around for a long time, but I've never personally bought anything through them. A quick google search didn't really throw up any red flags for poor business practices other than the owner was rude to one guy. Don't get this site confused with J-Spec Auto in Richmond, VA.