Get the DA21 batteries with the highest mAh rating you can find. Brand doesn't matter as long as it's name brand. Buying off eBay is risky. The 8 hour batteries you see aren't made with the same quality control standards as name brand batteries. I've ordered 6 total off eBay over time. They don't last for the full 8 hours advertised but they do last for several hours. The main thing I've found with those batteries is outside case often is a tiny bit too large and the contacts inside the battery might not make full contact with the terminals on the camera. I got two of those from one seller that wouldn't work unless I firmly pressed the battery upwards to make contact. The outside was too big and made the contacts sit so low that they were unusable. I have three now that if I press down on the battery it will lose contact but as long as I don't put too much force on them they work fine and provide a fairly long running time for $20.
So the moral of the story is, in this case at least, stick with name brand unless you like to gamble.