Background or "reference" sources are a great place to start in building your bibliographies. These are scholarly encyclopedias, handbooks, bibliographies, and similar sources that will often contain lengthy essays with background information on a topic and overviews of the relevant scholarship. Citations to the secondary literature will always be included, and often so too will citations to primary sources.
Oxford Research Encyclopedia in American History
This is a large collection of bibliographic “articles,” each containing numerous bibliographic annotations, often of both primary and secondary sources.
Gale Virtual Reference Library
Includes the International Encyclopedia of the Social Sciences and other sources useful for quick references.