Buying Copies of WOW

Hello,

I was considering starting to multi-box. I already have one copy of WoW w/ TBC. I was wondering what the most efficient/inexpensive way to get another Authentication Key is. Is there any way to pay for a WoW/TBC Key online? Do I need to buy 2 keys: 1 for WoW and 1 for TBC?

Thanks,
Dio

Dio, right now Bliz is offering the Battle Chest - WoW+BC for $40. They are currently sold out, so I went to the local store (Fry's) to get mine.

One note: Make sure to send yourself Test Account keys FIRST. This gets you lots of bonus days.