The best solution is to go to: http://us.blizzard.com/store/?rhtml=y

Type Authenticator, into the search box.
Select the US or Canadian/Australian option.

They're both in stock.

Once the authenticator is associated with your account or B.Net email, no changes are possible without the code.
The code changes every 30 seconds, and is six digits long.. so one in a million chance of guessing the right code.

You can associate the authenticator with multiple accounts or B.Net accounts.

So if you don't care about the new Account Management features for having all your accounts on one B.Net account, you can use one authenticator for 5 B.Net accounts (with one wow per B.Net account), and still log into each game at the same time with one code.

Or alternatively have the accounts on the same B.Net account, gain all the new features they're implementing, and have the one authenticator associated with the B.Net account. Which means a 30 second delay between each account logging in. A small price for the security.

I personally prefer 5 B.Net accounts, and logging them all in at once, on one code.