Be aware that they do sell souvenier mugs at the parks, but they are NOT refillable. I got a really cool light up one a few christmases ago. They have a mug that they sell at MGM and some type of sipper at AK, but there are no refills.
Sea World has a program where you can buy the mug, and refill it all day long for 79 cents a refill. They are really big, too.