Interviews:
Interview knowledgeable authorities on the issue (government officials, union officials, businessmen, service agency representatives, etc.)
Personal Experience:
The essay may include your own personal experiences (ask your instructor if this is acceptable) that relate to the topic. Sometimes personal experiences can be the strongest examples in an argumentative essay. For example, a single mother with four children may be able to write cogently on the experience of being on welfare.
Although the thesis statement itself does not have the words "I think" or "in my opinion" in it, you may use the personal "I" elsewhere in the essay when citing personal examples.
Your essay should not just be "a patchwork of unrelated facts and quotations strung together one after the other....Use quotations to fill in and support your own presentation, rather than using your own writing merely to tie the quotations together" (Millward and Flick, Handbook for Writers 396-397). Too many citations of other works shows that you're not really thinking on your own; on the other hand, too few citations shows you're relying too much on unsupported personal opinions.