Where can I download the campaigns Blizzard made?

You mean the maps or your missing the files on Starcraft? o.o If your looking for the maps(to edit) some were posted like SC : BW Terran Mission 5 and such, I don't think you can download, also you should have posted this on Null in file requests, and if their not in SC then you better redownload Starcraft

No I think he means the Precursor campaigns http://games.softpedia.com/get/Mods-Addons/Starcraft-Precursor-Campaign.shtml