There are numerous gift shops that sell earrings in both parks. The biggest selection at MK will be at the Emporium and Uptown Jewelers, both on Main Street.
At Epcot, there's Mouse Gears in Future World. Some of the shops in the countries in World Showcase will also have jewelry from the host country. I've gotten a number of nice, inexpensive earrings in Mexico.
All of the monorail resorts have earrings in at least one of their gift shops.