As Bob does, I get off-brand ones because there are SO much cheaper. Here, for instance, are some off ebay:
$2.69 each with free shipping. Takes about 2 weeks from Hong Kong, Shanghai, or Taiwan but isn't that where all the ones at Walmart or Best Buy come from anyway?
Buy 3 or 4. Make them "A", "B", "C" and "D" and track their life (if you care). Toss the odd one that performs poorly. I don't care because at this low price and weight, I always have an extra battery along.
Despite advances in solar cells, in almost all situations, carrying 3-4 extra batteries will be cheaper, lighter and more versatile than trying to recharge on the trail. For thru-hikers, buy a dozen and include them in your resupply shipments and periodically mail old ones homes (or just throw away at this price). If you have extra juice for one leg, do some editting to thin out the culls.
This works for cell phones, cameras, and cordless home phones. Type a battery part number into eBay's search or enter the make and model of the camera and lots of options will pop up. If no one offers free shipping, search of "Sanyo XYZ-123 lot" and you'll see lots of 3 or 4 (and 10 and 100) pop up thereby reducing per time shipping.