Not sure if this is true or not, but a mechanic local here in Seattle said that they were on a complete USA backorder. I doubt if that's true though. Do a search here in Iboats forum and look at the postings under OMC Shift Cable and I bet someone's already given lots of sources.
I just picked up the entire assembly off of ebay including the shift housing. Cost me $500.00 tho.
if you need just the cable this place has it in stock:
I went for the whole assembly, i know it s agamble off of ebay, but i didnt wanna mess w the hassle of installing the cable into (a maybe faulty) shift box......