MacBookAir7,2 battery

Hi, I have to work with my MacBookAir7,2 plugged into power. The battery level drops to 80% and it turns itself off after about 10 minutes without power. I assume I need a replacement battery. Should I take it to Apple and ask their advice or should I just buy a replacement battery online? Sorry... probs a silly question but I've never had this kind of problem before and the darn thing is only just 3yrs old! Thanks :)