Many Canon 5D Mark II owners have struggled to find this battery in stock over the past year. They have been pretty hard to come since the 5D Mark II was released, and they might become scarce again soon since the Canon 7D also uses the LP-E6 battery.
At the time of this post, the Canon LP-E6 battery is in stock at Amazon, B&H and Adorama. Here’s the links to each store’s product page for the LP-E6.
If you’ve been putting off the spare for your 5D Mark II or if you’ve got a 7D on pre-order, it might be a good time to pick up a spare instead of waiting until the 7D’s ship out in force, which should occur over the next few days. Check Canon 7D availability here.