I believe that most of the third party programs companies vanished in the early '90s. Their products just kinda stopped selling - if you take the time, you may find a store that has a box of them tucked away somewhere that they'll most likely give you, or sell to you for a very reasonable price. Also, I believe that there's some of them (Kid Nepro comes to mind) who still sell their programs on the classifieds of Keyboard magazine.
dB