Usually there's always some candidates you can find in the tab that lists all individuals. Just set the gender filter to the opposite and turn all other filters off. Even during low times you can usually find a handful of candidates, even though they tend to be worthless. If there's still no eligible candidate at all you just have to wait it out. There's a pretty steady influx of individuals in the game so usually it doesn't take that long. Also, try to arrange betrothals instead of waiting for your heirs coming to age. So instead of scurrying to find a candidate when your heirs come into adulthood, keep an eye out for candidates from the day they're born. Sometimes good stats are trumped by diplomatic benefits.