I bought a cheap one from Janns Netcraft. Its not fancy but it works great for me. I think it was around $15. The jaws probably wouldn't handle throttle cable, but with a file or a dremel, you could probably modify it easily. Just file the jaws out to fit whatever size you need.
Finding a fly vise to hold the throttle cable is going to be challenging. It is about 1/8” wide which is larger than the average fly vise will hold. It would be significantly larger than a 6/0 saltwater hook. The largest jaws are made by DynaKing and Anvil. They will accommodate up to an 8/0 or 10/0 hook, which should do the job. They are NOT cheap. Some of the others might barely fit the cable but it would probably collapse under the pressure of the jaws or squirm around.
You could make one out of a small rotating jewelers vise off ebay. Something similar to the one of the above poster, however few of us have access to a welding machine.
The one Dick Moore at Moores Lures sells might be the way to go. He is a real meat and potatoes type of guy and I would imagine it would not be overbuilt or overpriced.
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