Below are some general resources for finding sources for Ancient History. However, there are also dedicated sections for:
Fordham University provides a list of links to online primary sources from the modern era.
An online academic database used by universities worldwide that is a superb resource for History students. To view full articles, you will require membership.
This searches internet sites for academic articles and books. It often only provides one or two pages, but it is often enough to find quotes and bibliographical details.