Samuel L. Jackson's celebrity voice can be purchased for a one-time fee of $0.99 (later, it will go up to $4.99). There are two options for downloading and enabling his voice:
1. Purchase through your Amazon account
Preview and purchase his voice on the Amazon listing. The listing also has a great video showing additional features.
2. Ask Alexa to purchase for you
Say, "Alexa, introduce me to Samuel L. Jackson" to any Amazon Echo device.
Letting Sam Jackson use explicit language is optional but recommended. :) To do this, simply open your Alexa app, navigate to Settings, and toggle between clean and explicit content.