Hi Rob
I haven't used any myself, but have only seen Yongnuo ones other than Canon of course. Whether or not they are suitable or reliable I can't say.
http://thephotogadget.com/de/content/yongnuo-sf-18-flash-battery-pack-canon-and-yongnuo
Andy
How many pics are you taking? I used high capacity rechargables, 2 fresh sets and rarely do I change to the 2nd set, (on a 480ex though) about 1000 pics on a set.
That's the one I've got Rob and it works well. I used it at Whitby Goth Weekend and again at Venice Carnival, using flash for every shot I didn't need to change batteries. At each weekend I took over 1000 shots. I use the Duracell stay charged rechargeable batteries and have a set in the 580 as well.