the newer '89-'04 pipes will work on the '86, with very little modification....meaning you'll most likely need to move a mounting tab, or two around...to get it to mount up just right.
other than that, you'll be lucky to find one on e-bay/craigslist.
then there's this option...
http://www.kxriders.com/forums/index.php/topic,8164.0.html there's been a few members that have used Custom Pipes Inc. (CPI) for older bikes that pipes are no longer available....and they look great.