I think you are approaching this the wrong way. The first thing you need to do is figure out what the newsletter is going to be about and then you can find the right type of business partners for sponsorship. They should be at least tangentially related to your newsletter topic.

You talk to them directly, show them the documentation, and say this is what I am owed. If you aren’t going to pay by X, then I will file a lien on the property. Calmly, with confidence. Make it their choice on what happens next with clear actions.