VolunteerMatch claims to be "the largest network in the nonprofit world, with the most volunteers, nonprofits and opportunities to make a difference." You must create a free account to search the listings, but there are several advanced search options for finding the right opportunity.
Idealist is a job search engine that focuses on jobs, internships, and volunteer opportunities in the non-profit sector with an emphasis on social and environmental responsibility. According to their site, "Idealist connects millions of idealists – people who want to do good – with opportunities for action and collaboration all over the world."
Some departments offer "Special Study" courses which will allow you to develop your own service learning opportunity as a for credit course.
Also some departments offer classes that provide work/service learning opportunities that may not appear in the Class schedule search (Examples: BA 404 - Small Business Consulting and ACCTG 409 - Field Study in Taxation).
Read through the courses in your department to find these opportunities. They can be found in the link above under "Part 5: Individual Departments - Courses and Curricula."