I 2nd Cablemod, their pro line are really really awesome. Kind of pricey but beautiful and top quality. Do you want extensions or a full replacement set? Cablemod makes both, and they have replacement sets for most brands of PSU.
If you're going with extensions, one thing you might want to consider is how much room do you have in your case for the cables. Regular cables + extensions will take up a ton of room and depending on the case might make things too tight. Ensourced.net is another good place for custom cables, and they make what they call Shorty cables, which are 3 inch long. IMHO if it was my case with an extension set I'd pretty much need the shorties so everything could fit. You could spend a little more and get a custom extension set from Ensourced, I've never used them but they're highly regarded in the modding community.
Cablemods kits don't come with all the cables you need but you can buy them separately on Amazon. I'm currently on their waiting list to get a full set of custom cables made. Their non pro cables are still a lot nicer than most of what you'd find out there.
Their cables are seriously about 5 times nicer than the set from Bitfenix I bought about 6 years ago. I went cheap and wished I hadn't. Not that the Bitfenix set was bad, but Cablemod is just on another level and worth the premium.
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