I've bought my water aerobics wear from the Junonia catalog/website. junonia.com.
To ensure easy movement (no uncomfortable pulling), I suggest buying separates--for example, I wear their swim briefs along with several of the racer-back tops that I alternate from week to week. That way, when you're doing vertical arm movements, your shoulders don't feel weighted down by pulling against your bottoms, and--on the other end--you never feel like you're going to suffer a wedgie.