Publishers are also (usually) particular about what kind of advertising they run in their emails. They want to make sure that it's "on brand" because if it's not then people might stop opening or reading their emails.
It's the classic problem of the 2 sided marketplace but the easiest way to start out here (IMHO) would be to find a newsletter or two and go out there selling their inventory. When you're oversubscribed go and find another relevant newsletter and sell their inventory. Wash, rinse and repeat.