A Descriptive Catalogue of the Government Publications of the United States, September 5, 1774-March 4, 1881, compiled by Ben Perley Poore.
Locations: Government Documents - Reference - xx Z1223 .A1885 1962 and Main Library Reference xx Z1223 .A1885 1962
Checklist of United States Public Documents 1789-1909, Congressional: to close of Sixtieth Congress; departmental: to end of calendar year 1909, compiled by Mary A. Hartwell.
Location: Main Library Reference - Encyclopedias - J83 .C16 1990
Checklist of United States Public Documents, 1789-1970: Indexes, compiled by Daniel W. Lester and Marilyn A. Lester.
Five volume work
Locations: Government Documents Z1223.Z7 L45 - x and Main Library Reference Z1223.Z7 L45 - x
Cumulative Title Index to U.S. Government Publication, 1789-1976, compiled by Daniel Lester, Sandra Faull, Lorraine Lester
Location: Government Documents - Reference - Z1223 .Z7 C8 - No Loan
Comprehensive Index to the Publications of the United States Government, 1881-1893, by John G. Ames.
Location: Government Documents - Reference - Z1223 .A125 - x
Document Catalog, 1893-1940
Location: Government Documents - Reference - Z1223 .A13 - x
Cumulative Subject Index to the Monthly Catalog of U.S. Government Publications, 1900-1971
Fifteen volumes provide subject access to the Monthly Catalog.
Location: Main Library Reference - Encyclopedias - Z1223 .A18 Cum. subject index
Ancestry.com is available for free to guests at Port Jefferson Free Library. Contact your local public library to see if they have it. Many do. You can also do a 14-day free trial. (You will have to use your credit card to set up the trial and you must cancel it before 14 days or they will begin to charge you monthly.)
You can also search on the internet to see if a specific state or municipality has made census records available online.
Also, try: The USGenWeb Census Project - a national Census Project that has many years and counties online.