I have a newsletter in the niche of meditation and spiritual music with about 2500 people - we use some affiliate links in the newsletter but they don't really bring in any revenue - how do find companies that would want to pay to be featured in our newsletter. 2500 might not be that many people but the niche is highly targeted. Also our youtube channel has just under 200,000 subscribers.
I have a lot of experience with email marketing. More than I wish I had sometimes!
2,500 is on the small side, yes, but it doesn't mean there isn't value there. As an advertiser, I've had success with smaller lists, it all depends on your historical performance rates, and what the offer is.
Is your list growing? What are your average open rates?
It's probably too small of a list (now) to start effectively selling to advertisers, but you might want to try some ad networks who specialize in email marketing.
Here are two:
https://liveintent.com/
https://www.syndicateads.net/ - they do email sponsorships, as well as other types. This company is owned by https://www.buysellads.com/. They would sell sponsorships for your newsletter on your behalf.
To grow your list I recommend using one of the following solutions:
http://optinmonster.com/
https://sumome.com/
Hope this helps!