You can also check with the alumni association at your high school or college. You never know who in your school may have access to opportunities within their company. This strategy may also give you an internal sponsor who would be able to speak on your behalf for job opportunities.
Local Community Institutions - This could include religious organizations or the local Boys and Girls club.
Trade Groups - This can include the National Association for Accountants, SHRM - Human Resources Professionals, and many others. There is at least one major organization that pertains to your career or interest, so do your research.
A couple of key pointers once you have attended a networking event:
Always follow up and keep communication lines open
Send a "Hello" e-mail every 3-6 months letting others know what you are up to
Create your own networking event and invite them to other networking events
Keep your resume up to date. You never know when someone may have an opportunity for you that you can apply for
Keep good files of people you have met, his/her expertise, and industry they work in. This is ideal if you are considering switching professions
Good Luck and have fun networking.