Target carry a range of very life-like artificial plants if you want to check them out before purchasing. Amazon is one of the most reliable on-line sources of good quality ones. Check out the net for retailers in your area.
All living plants need to perform photosynthesis to live. This process uses light energy from the sun to convert carbon dioxide and water into glucose (sugar). The water is taken in through the roots and the carbon dioxide enter through pores in the leaf called stomates, which can open and close. For carbon dioxide to enter a cell, it must first dissolve in water. Hence the cells inside of a leaf are covered in a thin film of moisture and the air around them is 100% humid, the same arrangement as the cells in our lungs. It is only the cells on the outer surface of leaves that are covered with a waterproof layer (the epidermis).
Stomates open in sunlight to allow carbon dioxide inside the leaves. However, if the air outside the plant is less than 100% humid, then the plant will begin to lose water to its surrounds. Water loss gets to a certain level, the stomates close as a result. This incidental loss of water to allow carbon dioxide uptake is called transpiration.
All living plants transpire as they need to photosynthesise to live. However, plants from more arid or wind exposed environments have mechanisms to help reduce their rate of transpiration. Living plants also need moisture in their substrate to be able to take up water and a certain amount of minerals (fertiliser). And, of course, appropriate light. Growing plants in a terrarium is a skill in self. If you wish to give it a go I am happy to provide assistance. One live plant done properly is not going to cause a significant increase in humidity and may even help.
What reptile/s are you hoping to put into the enclosure?
