Online, B&H Photo (bhphotovideo.com), Adorama (Adorama.com), and Amazon.com will all have them in stock.
Dealers will sell 3rd party batteries as well as the genuine Canon battery so check the details to verify that you are buying a geniuine battery (these are all reputable sources -- they wont advertise a genuine battery and send you a knock-off.)
Also... batteries do not like to sit unused for extended periods. I own multiple batteries but they are all labeled so I can make sure I'm rotating them. This avoids constantly using the same ones over and over and ignoring others.