Are you looking for MPH employment at the federal level? The United States government runs a useful site called USAJobs to help you find such openings. Search using keywords like “public health” to find what is available. You can modify your search based on whether you are already a federal employee, where you would like to work, and what kind of salary you are looking for.
Check out Resources for MPHGrads to find other useful gems to help you make a difference in the world.