Real tweeters are more expensive, and you can find real tweeters that will fit in the same hole. Real tweeters may last longer and sound better.
Piezo tweeters can be purchased at radio shack or any speaker supply house. They also fit in the same hole. There is rarely any difference between a $10 radio shack piezo and a $40 piezo that you purchase from the speaker manufacturer.
So, why pay more for the same part?
You can order replacement from the manufacturer, but it will cost several times more.