Hi Sara!
It sounds like someone in your family will be celebrating their graduation in Walt Disney World. What an awesome way to reward them for their hard work!
In the past, I've definitely seen the graduation-themed mouse ears in the gift shops. The thing I really like about the merchandising at Walt Disney World is that they are constantly introducing new products. That being said, however, there's no guarantee that you'll see something again that was available for sale during a prior visit.
You have a few options to consider:
Option #1: As the graduation season approaches, you can check online to see if they're available yet.
The Disney Parks merchandise site regularly updates their stock, so you might find some there.
Option #2: The Chapeau is my go-to store for Mickey Mouse ears. It's located in Town Square, right as you enter the Magic Kingdom. They have a great selection of mouse ears.
Option #3: If options 1 and 2 don't work, ask a cast member either at one of the gift shops or at Guest Relations to help you track down a set. They have a tremendous inventory system that allows them to see which shops around the property have a particular item. For example, if you see a shirt you like in Epcot's Mouse Gear shop, but they don't have your size, a cast member can look the product up and see where else it is being featured and whether your size is available at those locations. Pretty neat!
I hope that helps and I hope you have an amazing vacation!!