The stock cable is P/N 15637721. Has it been discontinued, or is it just rumored that it has been? It may be very expensive, but would still be cheaper than mods required to fit a Morse cable.
The C4 Corvette uses the P/N 10219967 for shifter cable for all years 1984-1996. I do not know if the Corvette cable will fit a SyTy or not, but it could be worth investigating, as it probably is still available. A lot more C4 automatics were produced than SyTy's.