Recently did exactly the same thing as noted on this post... I went to the Technical Care Center in Hyatt plaza (mentioned by txfugi above), and they had power supplies for both the older style power brick, and also the newer "slim" style, which is what I wanted. They didn't have any official Microsoft ones, but their third-party one seems to be fine. One advantage to these over the original Microsoft power supplies is that they're 110-240 V, so we won't need a transformer in any country.
Recently did exactly the same thing as noted on this post... I went to the Technical Care Center in Hyatt plaza (mentioned by txfugi above), and they had power supplies for both the older style power brick, and also the newer "slim" style, which is what I wanted. They didn't have any official Microsoft ones, but their third-party one seems to be fine. One advantage to these over the original Microsoft power supplies is that they're 110-240 V, so we won't need a transformer in any country.