Research Guides: 

Locating Federal Regulations [pdf]
Locating Presidential Documents [pdf]

Databases: 

HeinOnline
Libraries include: Federal Register Library (vol. 1, 1936-vol. 51, 1986); and Weekly Compilation of Presidential Documents.

LexisNexis Congressional
Can search by keyword or citation for Code of Federal Regulations (1981 to current) or Federal Register (1980 to current).

LLMC Digital
The world's largest digital collection of legal literature and federal executive, legislative, and judicial branch documents in microform format.  The collection includes statutes, administrative decisions and reports of government agencies, boards, and departments. The current emphasis is mainly on historical documents (1790-2000).