They make their own. But if they are empty you can add a little rainwater or distilled.

You shouldnt add water to the pitchers because it dillutes the digestive enzymes, itll make its own "juice"

The plant will maintain an equilibrium of liquid. You can add a cup of water into a large pitcher and the next day the level should be how it was before filling. Water added to pitcher doesn't affect nutrient absorption or reducing digestive enzymes.

But one should NEVER add fertilizers to the pitchers. This will disrupt the natural bacterial enzyme action already present in each pitcher. Adding nutrients where it does not belong is a way that pitchers can die prematurely and plants dying. Nutrients made for roots should be absorbed by root action. Pitchers can only absorb nitrogen from the bacteria present in them. They have no way to absorb trace or micronutrients present in a pitcher.
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A vigorous N. ventricosa growing at the top of a fifteen foot citrus tree. Note the uppers while green, are still nicely sized and doesn't affect growth of the matured plant.