LetterHunt - LetterHunt connects you to 10,000+ niche, active newsletter writers to promote your product.
Sponsor This Newsletter - 100+ newsletters ready to promote your product
Newsletter Operating System - All-in-one dashboard to launch & grow your newsletter
Newsletter Crew - Podcast, blog, and community for newsletter creators.
Product Promotion Stack - Collections of Free resources to promote your product or startup.
Kickstart Side Hustle Newsletter - Dwarf-size newsletter with viral marketing case studies