If you are interested in government, arrange a meeting with your local elected officials or their staff. They have access to internships only offered to students from your university and also have access to alumni. Visit your school's career library and browse internship guides. Many publishers, such as Peterson's, publish yearly comprehensive directories of national internships. The internet is an excellent source of possible internships.
Availability can change monthly, weekly, or even daily, so start searching today.
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.