Bluebook R18.2 says you must collect all sources in print, an exact copy of the print, or an official or authenticated online version.
Exact copies are usually published in PDF format but are sometimes published as images (e.g. TIFF, PNG) or microform (slides, called microfiche, or rolls of film, called microfilm.)
Unfortunately, Lexis and Westlaw generally assert copyright over both their secondary sources and (controversially) their versions of statutes, cases, and other primary sources. With a few exceptions, they usually do not provide scans of their sources and block others from doing so, requiring you to use print or microform.
However, the library subscribes to many other databases that do provide exact copies of print sources:
Additionally, some trustworthy free websites provide exact copies of print sources:
Official and Authenticated Sources
Increasingly, government websites post online versions of cases, statutes, regulations, and other agency materials. Bluebook R18.2 allows you to cite these online versions if they are:
Some government online sources are authenticated but not exact copies of the print sources, such as the federal and California statutes. In theory, Bluebook R18.2 should allow you to cite an authenticated or official online source even if it is formatted differently or published on a different schedule from the print version. However, if the format of the online version differs substantially from the print version, it is safest to use the print or an exact copy of the print.
Most cases, statutes, and other primary sources are published in competing editions by the government and one or more private publishers. Bluebook T1 tells you which publisher's version to collect for each source.
Usually, the Bluebook will tell you to collect the official government published or authorized version if there is one. This includes the government published or authorized versions of the federal statutes (U.S.C.), federal regulations (C.F.R.), U.S. Supreme Court cases (U.S.), California regulations (Cal. Code Regs.), and statutes and regulations for most other states.
However, for some sources, there is no official government published version. Most notably, California traditionally did not publish a print version of its statutes and, although it now posts its statutes online, it has not designated the electronic version as official. As a result, T1 tells you to cite the unofficial Lexis and West published versions of the California statutes (Cal. <subject> Code.)
Finally, the Bluebook requires you to collect most cases from West's National Reporter System, an unofficial set of reporters that cover almost all federal and state courts. Although many states (including California) have official reporters, this rule makes it easier and cheaper for law firms and libraries to buy print cases because they only need to buy one set of cases from West instead of potentially buying fifty different sets of cases from fifty different state publishers.