You can buy the icing, but it doesn't taste homemade, so I'll share my easy Key Lime Buttercream Recipe.
1 cup (2 sticks) unsalted butter , firm but not cold
1/8 tsp. salt
3 1/2 cups confectioners' sugar, sifted
1/2 tsp vanilla extract
1 Tbs + 1 tsp. key lime juice
1/2 tsp. milk
Follow the directions on the box to bake the cupcakes. Once the cupcakes are cool, poke the middle and add a couple of drops of key lime juice (I like Nellie's) to the center of each cupcake.
Beat together butter and salt on medium speed until light and fluffy. Reduce mixer speed and slowly add confectioners' sugar; beat until well combined. Add vanilla, 1 tsp. key lime juice and milk; mix until just blended. Do not overmix or frosting will incorporate too much air. Frosting consistency should be dense and creamy, like ice cream. Frosts over one dozen. Put frosting in zip loc bag and snip a corner off. Use the bag as a pastry bag and swirl the yumminess onto each cupcake.