here is some sort of an adapter, http://www.everythingusb.com/20-pin-usb-3.0-header-connector-21232.html
I am not going to look for another type, but if you bought this you would also have to purchase 2 usb 3.0 cables to run to the back of the mobo.
I would highly advise simply purchasing a mobo that has usb 3.0 headers onboard.
the gigabyte is the best deal.
There might be other cheaper alternatives to make your chosen motherboards work, I just don't know any or can not find any.
If I was in your shoes I would opt for the gigabyte. I personally do not use usb 3.0 in my case because I have no need for them.
side note- newegg has a 25% off on lian li cases, if you are curious about other cases.
Best buy has an antec 300 for 35 bucks, this is a great case.