I've found two different Black Panther comic freebies! First up are over 200 old school Black Panther comics available for free on Comixology as digital copies. This batch of comics includes their entire library of single-issue Black Panther comics.
For something a little newer, Amazon is giving away all 22 issues of the ongoing Black Panther current run by Ta-Nehisi Coates.
From Book 1: A bold new direction for the Black Panther! For years, T'Challa has fought off invaders from his homeland, protecting Wakanda from everything from meddling governments to long-lost gods. Now, he will discover that Wakanda is much bigger than he ever dreamed… Across the vast Multiverse lies an empire founded in T'Challa's name. Readers caught a glimpse of it in MARVEL LEGACY #1. Now find out the truth behind the Intergalactic Empire of Wakanda! Ta-Nehisi Coates welcomes aboard fan-favorite artist Daniel Acuña for a Black Panther story unlike any other!
These freebies are presumably a tribute to Black Panther actor Chadwick Boseman, who recently passed away in August. I'm not sure how much longer either freebies will be available.