Annual park passes can be purchased online through US Park Pass at:
https://shop.usparkpass.com/collections/popular/products/america-the-beautiful-national-park-pass
Additional Information:
There is no digital version of the pass available, it’s a physical card that you must present when entering the National Parks.
You won't be able to use a purchase confirmation or receipt to enter a park. The physical pass needs to be presented when entering a park.