I would first determine if you REALLY need them. To me, they are only practical on a mountain bike if you are attempting long sustained climbs. I ended up removing them from my first mountain bike, because I kept "hooking" vines and other vegetation - this would cause the front wheel to turn sharply and I would crash.
If you do need the bar ends, DO NOT buy a new handlebar. Just loosen the brakes and shifters, and move them closer to the center of the handlebar (by whatever distance the new bar ends take up).
Moving your existing grips might prove to be difficult though. I've had luck in the past by laying the bike on it's side, pulling back the edge of the grip, and pushing something thin between the grip and the bar, then pouring in a little rubbing alcohol. The alcohol will help lubricate the grip, so you can push/twist it closer to the center of the handlebar. Unlike soap or oil, the alcohol will evaporate and leave no residue.