I have seen aftermarket one's out there, just not sure if they will fit your application...
The one thing I would watch is what your ripper bar (bar that holds the shanks) is rated for, or what load it will take. Going with longer shanks will put greater pressure on the bar and unit, which it may or maynot be able to take.
You may want to post a pic or the design to see if they are like others or aftermarket ones that are available.