Gateway Laptop Battery?

Last week I went to turn my laptop on and it wouldn't come on. I plugged in the charger and noticed it had a big "X" over the Gateway Laptop Battery symbol. I kept it charging for a few hours prior to me turning it on but now all of a sudden its saying I need to replace my Laptop Battery and this laptop is only a year old! What happened? Where can I get a cheap battery like Acer Laptop Battery?