If you want your iPad to read SD cards, you need the iPad Connector!

iPad 2 / 3 Connector
 

If you just want it to be able to put an SD card with some media like movies and music on it, and have the iPad read it, you need the iPad Connector, click below for more details:
 

http://www.fancygearbox.com/products/products_electronics/products_ipad_connector/products_ipad_connector.html