The DemocRATS are burning them as fast as they can get their hands on them.
I have been in a lot of bookstores, and I don't think I ever saw 30 year old yearbooks for sale. Unless you went to the school, there's no demand for them. Think about it. Where's the profit in carrying something so specialized? And how often is it that people unload their old yearbook?
If you went to the school you may be able to read the yearbook at classmates.com. Her yearbook was available online at some point and now it isn't. Neil Gorsuch's Georgetown Prep yearbook can be accessed, but not Brett Kavanaugh's.