Generally, these organizations do not have large budgets and cannot afford to pay an intern. Sometimes, interns are offered paid part-time positions with the organization if such a position becomes available. Please explore these opportunities. Usually, these employers are very willing to work with you so you can receive credit. Before you reject an unpaid internship, stop and decide why you were looking for one in the first place.
Some positions have both.
If you need experience in an area which typically offers unpaid internships, think twice before passing up the experience.