Public traded companies are required to disclose detailed financial information so that investors can make informed decsions.
Privately held companies are not required to disclose and therefore most figures for private companies are only estimates.
Mergent Online is the easiest source to use for finding company financials. It has the latest 15 years or quarters available and allows easy export to Excel.
ReferenceUSA has the most extensive coverage. Remember that figures are most likely estimates rather than actual.