I would say that any 3 cell lipo will work well, we usually suggestion a 2200-3000mah battery as it lets you run the gimbal for a couple hours before needing to swap the battery, but if you want a battery that will last a whole day of use you can get something bigger, but it just makes the rig heaver and more awkward to carry but its up to you. I usually prefer to just swap the batteries out.
As for a supplier Giantshark sell the better more reliable brands such as Gensace (which we will soon stock too). Hobbyking do sell super cheap batteries but many uses tent to suggest they don't last as long after a few charge cycles they start to drop in capacity , but then again the batteries are cheaper so its really up to you.
Hope that helps, and if you have any other questions just let me know