@Hal Cranmer I agree with @David Sisson
Using tax assessor databases and local GIS, you can build your own list. GIS also gives you the ability to analyze the property. You can turn on "layers" to show you different information about the property like a flood plain, topography, wetlands, etc.
I would search for either your county, state or city + "GIS" in google and you should be able to find a GIS tool to use. Some states/counties maintain better GIS systems than others while some have no GIS.
Use the GIS mapping tool to scan for properties. Click on the ones that look good to see more information about them such as what the zoning is and how many acres the properties are. The GIS should have landowner names and potentially landowner addresses which you can mail to or find phone numbers to cold call. Compile that data into a spreadsheet for mailing or cold calling purposes.
You can buy GIS software that makes it easy to do but they are expensive. I recommend bootstrapping it if you are marketing to landowners on a smaller scale
Let me know if you have any other questions or need any help.