My instinct is that we should always have around 40 active moderators. Some will drop off as life intrudes, so it's a good idea to add new ones from time to time.
One was is to invite them from the top rated member list. This is based on volume and popularity of posts and comments. If someone is regularily on the top of that list, it's a good sign.
Ning will be adding badges soon, so hopefully we will have other ways to recognize strong contributors, too. ("Noob Ninja"!)
Any other suggestions of ways to find good moderators?