Hmmm,
Not sure if they were ever available as a stand alone item. I thought you actually had to buy a new back housing.
Doug (KC5EIB) should be able to answer this one if he is still at work. Shows he is online right now as I write this.
RSU part number 12410809 for the Pro-96 CASE ASSY REAR will cost you $17.69 plus shipping and is in Stock
Description - NON REPAIRABLE CONSISTS OF: CASE REAR GRIP (L)/(M)/(S) LABEL MODEL TAPE ADHESIVE FOR GRIP(L) TAPE ADHESIVE FOR GRIP(M) TAPE ADHESIVE FOR GRIP(S)