It is always best to just use the original charging cable, as if there is a problem with trying to use another that either wasn't manufactured for use with the Pro Controller, or is just a third-party product, that could just void the warranty on the controller and/or Wii U console and any repairs wouldn't be covered.
And while Nintendo's Online Store doesn't seem to have the Wii U Pro Controller USB charging cable for sale or if they did, it is not in stock at the moment, I am sure soon enough they will start to sell them separately so that if you ever do need another one, you can purchase it directly from Nintendo and be assured that there will be no problems using it.