To my knowledge, Horton and Robertson are both still available.
Horton web site is
http://www.airsport.com/hortninc.htm
Robertson is called Sierra Industries now, web site is:
http://www.sijet.com/rstol.html
Also, Owen Bell (Aviation Enterprises) has STOL mods. We created a web page for him on this site, since he has not gotten his up yet. URL is
http://www.337skymaster.com/aviationenterprises.htm
Kevin