The official ones keep going out of stock and are not cheap. Third party ones have been around for a while but do not communicate with the camera or charge on the official charger. Thanks to Switronix and their sister company Series 7 we now have a 3rd party battery and also a really excellent battery pack. Click on the photo to be taken to the site.
I do not know the exact prices but they are much cheaper!
The battery pack powerbase by Switronix sits under your camera (or wherever you want) and lasts up to 6x the length of your normal batters. That’s 3 times more than the batter pack from Canon. Click on the picture for more info.