Chlorella can be purchased in liquid form from amazon.com or directly from the retailers listed on Amazon's marketplace. You also may be able to find it at your local health food store.
Generally, the recipes on NutriLiving.com that call for chlorella refer to the powder or tablet form of this algae. Please keep in mind that most of the liquid forms of chlorella available are extracts and therefore much more concentrated than their powder or tablet counterparts. Be sure to reduce the amount of chlorella a recipe calls for if using the liquid version. The label of the bottle should give you an idea of an appropriate serving size.
Below is a link to the Amazon.com search for "chlorella liquid":