The ARRL has the 2016 – 2020 question pool here. I highly recommend the Gordon West Extra Class study material ($29.95). For those of us on a budget KB6NU has a 2016 No Nonsense Extra Class Study Guide ($9.95) and on his website indicates ” Dont’t worry, though, if you purchase this books, and don’t pass the test by June 30, 2020, I’ll send you the updated version for FREE!”. 

The Roanoke Valley Amateur Radio Club has Extra Class Amateur Radio Study video for the 2016 -2020 question pool. Be sure to watch all of the videos there.

The online tools at HamStudy and eHam are excellent resources. The Extra class exam is scheduled to change on July 1, 2020.  Just make sure you are study the material for the date you are planning on taking the exam.