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Just like everyone else has said, I believe the only way to get a new one would be getting a new copy of HG/SS, unless you could find one off of Ebay or Craigslist or some site like that.
im guessing after awhile they might start selling them by themselves. but i think they want the extra money of making you buy the game again if you loose it.
or you can try to find one for sale of ebay.
Sorry for the little bit of cockiness, but I think a lot of people forget that Nintendo does have an online store and don't bother to check the source first. But there's your answer!