There is no one section or simple search for ethnographies, because books in the library are organized by the region or people being described, not the genre of writing.
One technique is to go to Advanced Search and type "social life and customs" into one search box AND a word for a region or people, such as "American Indians" or "New Guinea." After you have run the search, look on the left column of the results page and limit the results to Books/eBooks.
Another technique in Advanced Search is to search for the term "ethnology" AND a region or people, such as "Chin?" (for China, Chinese, etc.). Again, limit the results to Books/eBooks.
To access a series of short ethnographies in the library, search for the phrase "case studies in cultural anthropology"
Finally, searching for "culture and customs" as a TITLE will result in a set of descriptive books that are not quite ethnographies but will serve the purpose of your Anthropology 135 assignment.
The Wintu and Their Neighbors by 
Women of Shadows by 
The Force of Family : repatriation, kinship, and memory on Haida Gwaii by 
Portraits of "The Whiteman": Linguistic Play and Cultural Symbols among the Western Apache
Never in Anger: Portrait of an Eskimo Family
Argonauts of the Western Pacific
Border Visions by 
The Sorrow of the Lonely and the Burning of the Dancers by 
The Javanese Family: A Study of Kinship and Socialization
Hunting Caribou: subsistence hunting along the northern edge of the boreal forest by The High Valley
The Mbuti Pygmies: Change and Adaptation
Unorthodox Kin: Portuguese Marranos and the global search for belonging by 
Tournaments of Value: sociability and hierarchy in a Yemeni town
Freedom in Fulani Social Life by Women and the Ancestors by 
Himalayan Herders
The Canela: Kinship, Ritual, and Sex in an Amazonian Tribe
Ethnic Identity in China: The Making of a Muslim Minority Nationality
The Mi'kmaq: Resistance, Accomodation, and Cultural Survival
A Bagful of Locusts and the Baboon Woman: Constructions of Gender, Change, and Continuity in Botswana
The Lipan Apaches by 
The Huron: Farmers of the North
