There are many places to find these types of sources.
Not everything is available online or online for free.
Many reference books will contain the text of historical documents, reproductions of maps, statistics, etc. These are considered primary even though they have been collected into a book. Reference books, however, also contain analysis of documents, which would be considered secondary. Use the parts that fit your information need.
The Merced County Courthouse Museum is also an excellent resource for local Merced History
Many of these databases also contain other source types, so make sure you use the limiters to filter results to the kind of content you're interested in.
Many of these reference books include a mixture of source types, including primary sources (text of historical documents, statistics, etc.), secondary sources (analysis of the historical documents, statistics, etc.), and tertiary sources (bibliographies, references, etc).
Newspapers are an excellent place to go for primary sources. Use this Research Guide for California Newspapers. This primarily for the Central Valley, although some of these papers are from Los Angeles and San Diego. These are published in English, Spanish, and Russian.
These are some examples of online primary resource collections.
There are many university and government libraries and museums with online collections of primary source materials. Find these collections using Google.
You can even add the words library or archive to the end of the words you enter in the search box to see if a library out there has a collection on your topic, historical period, or historical figure.
If you have a collection you want to share, contact the librarians.