I have seen a lot of questions raised on various forums about buying advertising in e-mail newsletters. MarketingSherpa recently published an excellent article on how to evaluate newsletter advertising purchases.
The only eyeballs you’re buying access to are the ones that bother to open the newsletter.. Plus, many recipients only open issues for handful of seconds to scan the newsletter or just the top headline. Your ad may get a microsecond’s attention if any at all.

How do you find the newsletters in the first place? Take a look at the major sites in the niche(s) you want to target. Many have e-mail newsletters. Google is your friend.