Also, when you say "'you will need 2 of them and not use one each of the spare PCI-e connectors." ...I didn't realise your link was converting two 4x molex pin plug to (6+2) PCIe ...so even though my card only needs 6+2 you are recommending plugging both cables in?
Sorry, The only ones I could see one ebay were the ones with double PCI-e, (which didn't add the clarity)
Plug in all 4 molex connectors, and plug in one PCI-e from each of the new cables.
That way 2 molex plugs power one PCI-e, thats much more up to the job.
As for quality, I had some where the cable was very thin and some had poor crimping, others felt more substantial. Fatter cables carry more current.
I would imagine a respected brand name could be better than unbranded? You may just have to grab a couple and use the best.