You can buy one from Dell. You can also check ebay to see if people have any up for sale.
The most important part about buying batteries is that you DO NOT BUY USED. A lithium ion battery's maximum capacity is pretty much dependent on its age. Its max capacity decreases the day it is used, up until around 24 months when it holds so little charge that it isn't useful anymore. Wherever you buy from, make sure you buy new.