For most UCI researchers, ProQuest Congressional is the best source for Committee Prints.
Congressional committees publish other works in addition to reports. These "committee prints" vary greatly in content, and may include directories, legislative analysis, statistical materials, investigative findings, historical summaries, situational studies, hearing transcripts, draft reports, draft bills, and confidential staff reports.
In general, committees view prints as internal publications that may lead to a formal bill. So a print's issuance may not be announced publicly. The publication process also differs by committee, and formats vary widely. Most lack serial numbers.
See Bluebook Rule 13.4. Example:
Staff of H. Comm. on Homeland Security, 109th Congress, An Examination of Federal 9/11 Assistance to New York: Lessons Learned in Preventing Waste, Fraud, Abuse, and Lax Management (Comm. Print 2006).