I've found Flippa Apps to be a great platform for selling apps - particularly if you have a stable revenue base that you can evidence. Definitely worth a try.

2,500 does sound quite low for an email list. That's not a criticism, just to say keep doing what you are doing to build up the numbers in the meantime. In terms of getting sponsors I think this is a case of good old fashioned sales prospecting. Spend the time building up a list of potential sponsors that you would love to have on board. Create a spreadsheet or add them into your CRM with as much info as you can about each company. Do you know the contact you should be speaking to at the company? Can you get their email address? LinkedIn can be great for this. Build a defined product offer so when you start speaking to these people you have a clear set of benefits to talk about and a defined pricing structure. From there create an email series to send out to these prospects but be aware that you will NEED to follow this up with telesales activity to close the deal. Don't expect success on the first call but a consistent approach over a few weeks should start to wield results. Good luck.

If you are getting a 20% open rate but no good conversions then I would suggest it is the content within the email that's not working. The strong open rate means your subject line and general trustworthiness as a sender is not a problem, but your content just isn't hitting the right angle. Based on what you've said about the content of your emails it seems to me like they are all quite price-led messages (offers, coupons etc) but maybe that's not the best way to go. Maybe you need to find another angle or a set of angles that will convert better. These customers haven't signed up straight away so that means they must have unanswered questions about your service that's preventing them from making a decision. What are these questions? Find out and build a content stray around this. Or maybe go for a softer content-based approach rather than going straight in with pricing offers. Build credibility first over a longer series of emails and keep the pricing emails to a select few throughout the series. Hope this helps.