The Library of Congress Congressional Research Service does research and writes report specifically for Congress. These reports are not normally distributed to depository libraries like Alderman Library at UVA, so even we do not have all of them. But there are ways of finding individual reports.
NOTE: CRS reports can also usually be obtained by contacting one's Representative or Senator. That usually takes about two weeks.
Here are helpful guides for finding CRS reports on the web:
Here are helpful links for finding specific CRS reports: