The National Archives maintains copies of other censuses on microfilm or in bound volumes in Washington, D.C. Copies are also available in regional Federal Records Centers, plus it's possible to borrow microfilmed records from them through a local library, a recent development that has greatly enhanced the accessibility of census records to researchers all over the country. The Genealogical Society of the Church of Jesus Christ of the Latter-day Saints in Salt Lake City also maintains complete copies of federal census records, which makes them available through its branch libraries, and in the Schomburg Center for Research in Black Culture, a division of the New York Public Library.
Other libraries and historical societies around the country also have complete or partial collections in their collections. Large libraries, for example, often have copies of the records of their own state. The Microform Reading Room of the New York Public Library at 42nd Street, for instance, has the census records of New York State from 1790 to 1880, all the surviving census records of neighboring New Jersey from 1830 to 1880 and those for Connecticut from 1790 to 1810 and 18S0 to 1880. They also maintain the Soundex indexes for the 1880 census in these states, and there are scattered additional census records on film and in print and a variety of census indexes. The catalogue for these materials is maintained by the Local History and Genealogy Division of the New York Public Library.
A major problem you'll run into is that traditionally libraries have arranged census records geographically, by address, rather than by family name. The 1790 census has been completely indexed by surname, and the 1880 and 1900 censuses have been partially indexed by Soundex indexing, but in order to search for ancestors in the records of censuses conducted between 1800 and 1870, you need to know at least the county in which your ancestors lived during the census year. If they lived in a large city, you'll also have to know the ward in which they lived. Thus, maps and directories are extremely important for searching census records, especially in larger cities and towns. You'll find E. Kay Kirkham's Census Searching in Major Cities, which contains maps of the major cities in America in census years, a valuable tool.
Fortunately, private publishers have issued a great number of alphabetical census indexes, and soon you can look forward to when all census records will be indexed and the entire task of census searching greatly simplified. One of the major firms in the field is Accelerated Indexing Systems, which has been producing indexes, broken down into volumes by state and year, at an incredible rate.
Just as in using any secondary source, the researcher using a printed census index must be aware of possible errors or omissions. Because the original documents are often difficult to read, the people preparing indexes can easily confuse vowels or initial letters, substituting an H for a K, for example.
Despite such errors, the census indexes are invaluable. Considering the enormous scope of the indexing task and the poor quality of many of the original census records, the errors are understandable, but they mean that you must take special care when using them. If you're sure a family lived in a particular location during a census year but can't find them in an index, their name may have been misspelled or omitted. Using some creative thinking, you can check a variety of phonetic spellings. If that family member still can't be found, you'll have to check the original census records.
Finally, there are a number of Web sites which offer census records online. Begin with http://www.censusfinder.com, which offers free census records from 1790 to 1930. You'll find additional information and lists at FREE U.S. Federal 1790-1930 Census Resources for Genealogy & Family History. And for census maps, check out http://quickfacts.census.gov.