eBay is your friend! Search there for 'usb 3.0 20 pin male to female extension cable' to find many items like this, for as little as $4.99 with free shipping:
And if you search for '3.5" to 5.25" bay converter' you'll discover many variations on this device, for only about $3 plus shipping:
Good luck with finishing your build!
I received the USB3 externder cable that I ordered from ebay (blue, one male and one female end) and I attached it today so that the supplied cable from the supplied front bay card with two USB3 ports (from Intel, with my motherboard) would reach the USB3 header. After connecting, I tried the two ports with a USB3 compact flash card reader, and they work fine, very fast, so I'm happy to have the help I was given to find this extender piece(from ebay).