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.