Here's a list of webshops that distribute metal zines.
NIHILISTIC Distro:
https://nihilistic-webzine-distro.fr/Webshop/fr/14-zines-merch
TODESTRIEB Recs:
https://todestrieb.co.uk/collections/zines-books
INVICTUS Recs:
https://invictusproductions.net/product-category/zines/
SCHATTENMANN PUBLISHING:
https://schattenmanns.wordpress.com
