The Public Employee's Roundtable has an excellent list of websites for government agencies, states, counties, cities and government related associations. An organization or agency's website may contain current internship openings not listed anywhere else. If these are not any internship listing, it does not necessarily mean that there are no internship possibilities. Browse the staff listing and send a staff member a letter stating your experience and interest. Your fellow students may be engaged in internships and could point you to open positions.
Many organizations see interns as potential employees.
Your internship could lead to a permanent position when you are finished with your schooling. If you are interested in a federal career, now is a good time to begin. Federal internships are an excellent introduction to the workings of national policy. Often organizations may not advertise an internship, but could arrange one for you. Send letters of interest to organizations of interest.