As others have already said in the comments, the best way is to find a friend/neighbor/acquaintance that owns the game. You can move the WoW installation folder freely between machines and it surely saves time opposed to downloading 20 GB of data.
Another way may be to find someone that is willing to download the client for you. Downloading the client doesn't require an active subscription, just the registration to battle.net, and at the same time you can invite this friend to try the game with the 10-day trial.
Installing from the installation discs instead only saves part of the hassle, since the version available in the boxed version of MoP still has to be updated to the latest version of the game.