At Game Stop.
eBay or amazon
The old 'Nintendo Points' cards are for Wii and DSi only. Only the newest cards, which have Wii, DSi and 3DS on them, can be redeemed in th 3DS eShop.
In order to add funds on the Nintendo eShop, you can either connect your credit card and withdraw cash from there, or you can buy a prepaid point card which can be redeemed by entering the pin afterwards.
The content will go on your SD card.
It is already on the eShop for 3ds
Nintendo eShop prepaid cards can be bought wherever you can buy Wii U or Nintendo 3DS games. Example of this include Future Shop, Best Buy and EB Games. It is also important to ensure that the card you are buying is an eShop prepaid card. Wii points or Nintendo DSi points cards cannot be redeemed on a Wii U. On the other hand, older eShop prepaid cards branded as "Nintendo 3DS" prepaid cards can be used on the Wii U eShop.
The eShop for the Nintendo 3DS is out already.
Mario Kart 7 is already in the Nintendo 3DS eShop
The Nintendo eShop (also known as 3DS Shop) came out on June 6, 2011 in North America and June 7 in Europe, Japan, and Australia.
Using the Nintendo eShop.
Yes, the Nintendo eShop is free. All you need to access it is a working internet connection.
Firstly, you will need internet connection and your region settings must be in a location that has the Nintendo eShop available. Then, select the Nintendo eShop icon on the home screen. Enter your Nintendo Network ID and password to have access to the Nintendo eShop.