Blazichu said:
I didn't know that you can get a Jirachi using a special disk
Yes, in the United States, a month or two before Pokemon Colosseum was released, if you preordered Colosseum, you would get a bonus disk, it has a preview of the Jirachi Wishmaker movie, a Pokemon Colosseum preview, and a gift feature where you could download a Jirachi onto Ruby or Sapphire, and it would be holding either a Salac Berry or a Ganlon Berry. These disk aren't available now, unless maybe some out of the way store that doesn't do much business has some leftover. Generally, you would have to go on eBay if you wanted one now.
In Europe/Australia, people got Jirachi from Pokemon Channel, and in Japan, people got Jirachi from a promo at movie theaters when the Jirachi Wishmaker movie was released over there.