I found it on the eShop under the Redownload section, but it has an unavailable status. (Other DSiware I have downloaded say "downloaded")
Might be time to contact Nintendo?
Maybe they recorded over it, like a lost Dr. Who episode...
Unfortunately, as far as I can tell the system transfer wipes the target system. Though a googling brought me here, which suggests that you can transfer your Ambassadorship first, then the games, and only lose any duplicate games you own.
Good luck!