Assuming your original battery hasn't been abused (ie: drained dead or very low), you can probably just get another battery of similar size and wire it in parrallel (tie the two positives together and the two negatives).
6V wired in series (positive from one to negative of the other) would be nice option if your current battery was near the end of it's life.
Normally, you want to change out all the batteries at once because if one has problems, it tends to draw down the new battery and quickly wear it out, so if there are any indications that your existing battery isn't in tip top condition, replace it.
Google images of wiring batteries in parrallel and it should get you some drawings of how to wire it.