This keeps coming up on my thread list as not read yet. It also says someone named "cgherhger" was the last one to post, but I don't see it here. I'm hoping this posting might make it go away. Someone advised me that this last post (that I can't see) might have been removed by a moderator.
You didn't give a lot of info, so here's a vague answer:
If your case has built-in USB 3.0 ports (unlikely), the cable came with your case.
If your case doesn't have USB 3.0 ports, AFAIK no cable is available.
Many Asus USB 3.0 mobos come with a set of USB 3.0 ports (contained in a small a small box with cable attached) that can be mounted in your case's external drive bay, if you have one available. Check with Asus to see if you can buy one for your mobo.