The battery pack is called the Pixel TD-381. This is suitable for Canon speedlites. I bought mine from ebay, they cost around $55 - $65 so it's not a huge expense. You need at least 12 batteries for it to work properly but it'sbeen really good for me.


Here's a link to a good review of it, this guy also makes some wicked strobist gear.


]http://www.flickr.com/groups/strobist/discuss/72157622919317987/