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!