I just bought a BP4L battery from the "Grade-A-Battery" Ebay seller listed above and was so impressed I ordered a second one. Unlike some of the other cheap knock-offs, this one is the full 1500ma capacity. Others put a cheaper, smaller battery inside the casing for a BP4L and call it "compatible". This one is the real deal. I also found the contacts fit real tight, in that I had to spread them slightly on the first installation. This was better than the opposite situation I had with the Mugen Battery I purchased previously (which is also a good battery, but more expensive).

I like to have several batteries because I take my N810 on multi-day back-packing and/or XC skiing trips where I am without power but want GPS tracking as well as the entertainment I can get from the N810. I am trying to rig up a small solar powered battery charger for these things as well.