Where do I get a Lightning Adapter for my Apple Pencil to charge the tool?

One comes packaged with the Apple Pencil.

If you lose this, there is currently no way to buy a replacement as they are not sold on their own. You would need to contact Apple Support and enquire about a replacement or charge the Pencil by plugging it into the iPad's lightning port.

Use Apple Pencil with iPad Pro - Apple Support

You can also buy third party chargers for the pencil:

http://www.imore.com/apple-pencil-accessories

They did say they would sell these when they announced the Pencil last year, but so far they have not.