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Lokar is a good one but there are a few companies that make them for the Hot Rod market where they need specific lengths. I have a spare kit in my garage hanging on the wall since I have had a throttle cable brake away from home and remember what a PIA it was to replace it.
Just for fun I looked around and you are right, they are not available in a lot of Corvette Suppliers. EKLER'S says they sell the cable under part Number: 25-110970-1 Alt #:37836 and it costs $46 when they have it in stock.
EBAY also has a listing for it. Here is the information:
1985-1988 Corvette Accelerator Cable GM 10181278 and it is over $50 for a used one...
Good Luck and I will keep an eye out for the cables. If they are this hard to find Maybe I should get one as well just in case. The aftermarket ones will work just fine but they might look different.