Everbuying.com has 4-Pack 3400mAh Panasonic NCR18650B Protected Rechargeable Li-ion Batteries on sale for $16.79 - 18% Off w/ promotion code SD18650 = $15.96. Shipping is free. Thanks iconian
Note, you may save an additional ~10% when you register your account. When you register you will receive 100 EB points instantly, and by 'confirming' your email address you will receive an additional 10 EB points (mine didn't arrive immediately, but the 100 points did). During checkout there is a box to input the amount of EB points you'd like to apply to the purchase. Applying 100 EB points will deduct $2 from the cost making it $13.76.
Editor's Notes & Price Research
Note, you must be logged in to apply the coupon code to your cart. Because the batteries ship from China, please allow a few weeks for delivery.
Because they are li-ion batteries, they have to go by ship, so they take 3-5weeks to arrive. Being very quality cells, it's very worth the wait.
$17.59 + free shipping w/ coupon code SDBattery
a few things: these are genuine panasonic cells that were modded after market with discharge/overcharge protection circuits (it kicks in so you don't overly discharge/overcharge your battery) generally, these are considered are some of the best batteries ever made. these are the ones you should probably be using in your flashlights
i been using them for about 6 years, starting with older 2900mah/3100mah versions
please be sure to use smart charger with these and not a cheapie one that just pumps the juice
If you are in the market for a quality li-ion charger, Opus BT-C3100 V2.2 in available for $34 + free shipping [discuss]