I have a curated gaming content-related newsletter called The Gaming Pub (thegamingpub.com) and most of my initial users came from promoting on Reddit on relevant subreddits. However, most of them do not allow self-promotion, so it's not the best place to be honest.
I had a little bit of luck as well that someone shortly after I started the newsletter mentioned it here on Hacker News when on an Ask HN for cool newsletters that they followed and this brought a bunch of users as well at the beginning of my newsletter.
Today I barely do any active marketing for it, I believe mostly comes organically from word of mouth and also from newsletter directories/aggregators.