The trick to finding primary sources is realizing that there is usually not a one-stop shop for primary sources. You really need to know what you are looking for because this can dictate where you search. Some questions to consider:
It's also important to realize that you might have to search in multiple locations for sources. A good starting place for finding primary sources relating to USU, Utah, LDS and American West history is USU's Primary Source Resource list.
Special Collections and Archives (SCA) is home to items that tell the story of the history of Utah, Mormonism and the Western region of the United States. SCA also includes the archives of Utah State University and the Fife Folklore Collection. For more help finding primary source materials in the SCA visit the Finding Materials in SCA guide.