All Federal Grant Writing Agencies!
In each State in the Nation you will need to visit the individual sites for each Department that is a Grant Writing Agency.
You should also network with Leadership from those Departments. You will also need a close connection with your legislators as they can inform you of upcoming RFP's and other solicitations your organization may be able to take on.
I recommend subscribing to a Foundation Center website as well as seeking out similar agencies and their funding sources. Foundations can be found in a number of different ways but the least time consuming is a database site such as:
Many times you will find reference to Foundations within State and Federal agency RFP sites in newsletters and a host of different arenas.