Schools and alumni organizations sometimes have old yearbooks in storage or for sale, and many have scanned copies of yearbooks to view on their websites. Other sources include Amazon.com and eBay. Enter the school name and "1968 yearbook" in the search field at either site to find the yearbook.
Amazon.com has thousands of yearbooks for sale. Many are reprints made from scans of previously owned yearbooks and may show markings and signs of use.
Ebay is a popular marketplace for original yearbooks, and it has a huge selection available at any time. If a particular book is not immediately available, it is worthwhile to check back frequently, as the selection always changes.