Yes, you can replace the battery and, with most makes and models, you can find sources other than the manufacturer. And, non-manufacturer sources often sell at much lower prices. I suggest Amazon.com.
By the way, how long did your battery last when you first bought the computer? Do you often need to run it on battery or can you normally keep it plugged into a wall outlet? Have you tried running your computer plugged in and with battery out? If it works that way, using that way part of the time is a way to extend your battery's useful life.