I know of two popular people who sleeve cables, Zeal cables and Mimic cables. There might be more. But competition is a good thing. If you're better and cheaper, have cooler colors, more customization options, then people will come to you. I was looking at Zeal, but it's just too darn expensive. It's only $20 or so for the cable, but it costs $15 for shipping from Canada, and $35 for a USB cable is just something I can't convince myself to do.
I think competition is a good thing, and if you want to start, don't let the fact that someone else is already doing something similar stop you. Just like there's Walmart, Target, Whole Foods, and so many more grocery stores that all basically do the same thing.