I have the battery pack for my 580 mk11, and got it from Kerso
It is worth every penny if doing anything where you need almost instant recharge times - I got it for a wedding, but have also used it at enduro meets where it will keep up with a 30d shooting bursts of 4-5 frames at a time. However, it is a lot of money for what it actually is - some plastic and a cable in a pouch..!