Old copies of high school yearbooks are most easily purchased by contacting the yearbook department of the high school in question. Also look on yearbook resale sites such as ThisOldYearbook.com and OurClassReunion.com as well as classifieds sites such as Craigslist.org or auction sites such as eBay.com.
The yearbook department of the high school in question is usually the best place to contact when looking to purchase an old yearbook because they are the ones who created the yearbook and have the most interest in preserving records of past issues. If the school does not have any old copies to sell, then ask about the printer used during the desired year to see if it has any copies. Unfortunately, many larger printers handle a considerable number of orders each year and are thus unable to maintain an inventory of past yearbooks. However, some smaller printers may keep an inventory of older or recently published yearbooks, so ex students should always check with the printer when before moving to an outside retailer. Sites such as Craigslist.org and eBay.com sometimes feature listings for old year books, though listings are most likely to be from private sellers than any business maintaining a consistent inventory of yearbooks.