A good starting place for finding data is the Government Publications Statistical Data Page.
Some major data portals:
Note: some items are [UW restricted]
The first place to check for data related to a particular subject area is with your subject librarian. For a list of subject area guides (created by subject librarians), click here: http://guides.lib.uw.edu/tacoma/
Some librarians will have a tab on the subject guides titled "Data" or "Statistics". If you're not finding what you're seeking, each subject guide has contact information for your subject librarian and please feel free to contact them directly.
Another good resource for data is the UW's Center for Social Science Computation and Research (CSSCR). CSSCR is a computer resource center located on the UW Seattle campus providing facilities and support for social science departments at the University of Washington.
Yes, you need to cite your data, just as you would cite information you found in an article, a book, a webpage, or any other format.
Here are some resources to help you cite your data:
Sometimes you want to use data found on a website or in a PDF. Use these tools to transform the data into a more usable format such as a spreadsheet, text file or export into data visualization software.