If you’ve got a list of your own (even if it’s small) then the usual answer is ask them.
If you haven’t got a list then find out where your target market are online and tap into that.
That said, there’s plenty of people who want to be able to write (better) copy and (better) emails. I’ve put the word better in brackets because not everyone is even to the stage of having any copy or emails to speak of.
A membership where you taught those and offered critiques of campaigns would be my initial reaction. Some of your regular emails could even be the critiques if you had permission to do that, otherwise you could still apply the lessons in slightly more general terms.