I e-mailed Nokia USA earlier in the week asking about purchasing an additional charger (and to find out when the power keyboard would be available) -- this was the response:
We understand that you want to purchase a Nokia Charger for your Nokia Lumia 2520 and you want to confirm the availability of the Nokia Power Keyboard in the United States. In response to this, we would like to inform you that as of the moment, we do not have any information regarding the exact date for the availability of the Nokia Power Keyboard in the country.
With this, we can only suggest checking the availability of the Nokia Charger from your service provider, Amazon.com, other third-party online stores, or from third-party retail stores near your area.