http://www.dvdboxoffice.com has the game on its site for CA$49.95 (about £23). They're not the speediest company in the world, but they offer free shipping and a cheap price given what Nintendo Europe feel is a good RRP for GBA games. Still, I'm keeping a lookout as to where I can find this game as cheap as possible, so this may not be the best place to get it.
I also found an Online Advance Wars game. I'm curious as to how this works and would be interesting in trying to play a game of it against someone, once I re-find the URL and such.
Update: Link attached. You'll also need these maps to play the game, and may find a map creator of use.
Work in half an hour. Back to the role of standing around for money, I guess.