Worldcat is a database containing catalog records from libraries around the world. This is a great place to identify materials you might want to request on interlibrary loan, if our library doesn't own them.
The Alkek Library has a wealth of books and other resources in our online catalog which will be useful when studying the American frontier
This type of search will get you to both primary sources and secondary sources which will help lead to primary sources.
Here are some examples of subject headings- (using these as a starting point and then clicking on others that appear in the catalog for books of interest can be a useful research strategy)