A number of states provide a list of processors; we have collected these lists here. Some lists are only subsets (e.g. only state-inspected plants, not federal). Formats vary, from downloadable pdf to searchable online database.
USDA's Food Safety and Inspection Service provides a list of federally inspected processing establishments on this page: FSIS Meat, Poultry, and Egg Product Inspection Directory.
It is available as a pdf and also as an Excel spreadsheet. The spreadsheet is more easily searchable and sortable.
To find the Excel spreadsheets, look for and click on the "XLS" just to the right of of "Alphabetically by Establishment Name" and "Numerically by Establishment Number."
If you want to use the pdf and have a current copy of Adobe Acrobat, you can search pdfs for words. So if you are looking for a plant in Cincinnati, type "Cincinnati" into the search box, and you'll find all USDA-inspected establishments there. However, if you try to search for "PA" to find plants in Pennsylvania, you'll have to scroll through all instances of "pa."
Each entry is coded by basic type (slaughter, processing, import, and/or ID warehouse) and gives establishment number, address, and phone number.
USDA's "Know Your Farmer, Know Your Food" project has created an online search tool called "Compass," which draws from multiple USDA databases, including the FSIS Database described above. You can search for USDA inspected processing facilities by unchecking all data layers except "meat infrastructure" in the left column. Each pin will show a processor. When you select a pin, you will see the business name and address but not plant size, services, or what species they process.