I would contact your school first, and see if they have any back copies. They very well might. I know my school has copies from all the years. Or, check out the local library that the school was in. Libraries in the same district usually have a copy of each year, and see if you can buy it from them. And your last resort my be to do some research online like the person said in the previous answer. Good luck! :)
To find a copy of your yearbook from 1988 would take some research. Contact your school and see if you could purchase a copy through them or the alumni department. Some reunion committees will have several extra copies of yearbooks that can be purchased. There are many websites, such as classmates.com or reunion.com, that may be helpful in tracking down copies of your yearbook from 1988.