Instead of spending money on a cable the better solution is to buy a card that doesn't require internal power in the first place. There are several to choose from, see the list in my signature. The cheap option is an Inateck KT4004.
If you insist on your current card and a cable, it will depend on what compromise you want to make. You're going to have to either give up a hard drive bay by using a SATA power cable to one of the backplane board's HDD connectors, or you'll have to use a molex-to-SATA cable running all the way from the optical bay, which will probably need an extension cable too, and if you have two optical drives you'll need a y-cable molex adapter on top of that, all of which together will probably cost as much as just getting an Inateck KT4004 in the first place and not needing any cables inside at all.
