[iamrfixit]

You are not going to find a pre made cable that is the correct length, you would have to make it. Putting the little loop on the end is more difficult than it sounds, unless you have access to the correct tool to install the swage on the cable. That's the silver crimped connector that closes the loop. You will also find another one under the piece of black shrink tube where the cable meets the chain. I would not use cable saddle clamps they get a little bulky and too likely to catch on the rail. You can get the right part cheap on ebay; the standard chain/cable for a 7' opener is actually a leftover piece when you add the extension kit for a taller door, so there are lots of them around.