It's a standard USB A to mini-B cable. Make sure you don't get a "Micro USB" cable; the 'micro' connector is smaller than the 'mini' connector and not compatible.
You used to be able to find these cables literally everywhere - even 7-11 stores had them at the counter. Nowadays though, with most phones taking either Apple Lightning or Micro USB cables, you may have to go to a computer retailer to get a Mini USB cable.
That hadn't occurred to me. Lonnie, that's probably what that is. You want a cable that has the same kind of USB connector on one end and a small connector on the other end. The end like the one on the WiFi dongle, that goes into your computer, and the other end needs to match the port on the front lower right of the 536's front face.