Is there an adapter with AC/Wall Plug Input and USB output?

Trying to find essentially the opposite of an iPhone cable. IIm wondering if there exists an adapter that will allow me to plug a wall outlet into a USB port. If so, let me know where I can find it!

That doesn't exist.

Although it's possible to build such a thing, the output current would be extremely limited, to about 50 mA or so, even for USB I'd be very surprised if any existed.