A primary source contains information from a first-hand or original source. Some example of primary sources are autobiographies, diaries or journals, photographs, and newspaper articles from the time period, just to name a few.
When working within the Common Core, there are four types of informational texts:
You can find Informational texts in a wide majority of places.
The CC State Standards has an Appendix B which gives lots of examples of informational texts at grade-appropriate levels. Be sure to look at the Table of Contents.
The Library of Congress has a collection of Informational texts you can access for free.
Another place to look for informational texts is in Project Gutenberg. These texts are all in the public domain, which means you can use them freely without breaking copyright laws.