While I do not recommend using Wikipedia as a source for serious historical research, it can lead you to good resources available on the web, as well as in print.
Many entries on Wikipedia will have three sections that can be of use to you:
I. Notes - End notes for the entry that will books and articles used for the entry. Some will have links to online versions.
II. References and further reading - A list of recommended books and articles on the topic. Again, some will have links to online versions.
III. External links - Links to web sites, some of which will be from scholarly or governmental web sites.
Here are examples of these sections from the entry on New York Draft Riots:
The circled links are primary source documents available online.
The last two sections, References and further reading and External links also have links to primary source documents online: