There isn't a place to buy them. My kids have both been chosen, and they were 14 and almost 13 at the time, and although I love them, they don't fit the classic "cute" Disney kid definition. They were very attentive the whole time, and clearly were enjoying her work.
It is enjoyable to watch, even if you don't get chosen. I agree that she can be hard to understand. It takes a while to get the rhythm of how she speaks, but this is a big part of our EPCOT experience.
It's also only one person who does this, so there are many days that she doesn't work. I'd say she mostly works in the 1-6 pm timeframe, so if you are only there early you probably won't see her. I know there are entire weekends that she doesn't work, because she's not on the times guide at all then.