I don't have a clue where's the best place to buy them, but I also had many questions about how to prolong them.
If used at home, does it affect its lifespan if it's plugged in the wall all the time, or whether it's better to fully charge/discharge in cycles, or whether it does not matter if you start charging at half capacity... this place has lots of good info:
http://www.batteryuniversity.com/