There is a wonderful new store at Downtown Disney called the
Hanes Design-A-Tee Shop. This store has many style of tees for both children and adults and potentially thousands of different ways to personalize them. If you're willing to wait until you arrive at WDW, a visit Make-A-Tee could be a fun activity.
If you'd like to get shirts in advance of your trip, you can do this through the Disney Store online. The link to the Create Your Own part of the site is
HERE. There are many choices of Disney character designs and you can add text to make the shirts completely personal. These shirts can be mailed to you at your home, or you can input the address of your WDW hotel and have them mailed directly there.
If you're looking for a different sort of personalized gift to have delivered to your hotel, you might want to look at the site for the Disney Florist and Gift Basket Department at disneyflorist.com. They have a large assortment of gifts and will also be happy to customize a basket especially for you.
Have a great visit to WDW.