To obtain an original copy of a 1976 high school yearbook, contact the school and ask if it has a copy in its archives. If you are unsuccessful, contact the yearbook publisher. If neither can help, perform an online search.
Search on eBay for “1976 [name of high school] yearbook.” Some yearbooks are typically listed for auction, generally for seven days, and others are listed at a fixed price. If the yearbook is not available when you perform the search, wait a few days and try again. Alternatively, you can try various school-related sites, such as E-Yearbook and Classmates, that are dedicated to connecting members of various graduating classes.