One website that can be useful is Library of Congress (loc.gov), which has thousands of primary sources on tons of topics. One website that I find helpful with my students is the immigration collection (link below), which explains the experiences of immigrants in themes: such as why people migrated, tension, diversifying, laws, and the community today. I like these sources because students can use them to connect concepts among various experiences.
http://www.loc.gov/teachers/classroommaterials/presentationsandactivities/presentations/immigration/chinese.html
.