Many people count their five elements and think: "I lack Fire, so I need Fire." This is a common mistake.
A missing element means that element is weak in your chart. But if the missing element is your unfavorable element, not having it is actually good. Example: weak Day Master who avoids Wealth — lacking Wealth stars means less financial stress. What you really need is your favorable element (Yong Shen), not simply what's absent.
First assess Day Master strength. Strong charts need control/expression/drain. Weak charts need nourishment/support. Then identify which element is most needed. This requires reading the specific chart.
Adding an element isn't just wearing colored accessories. More effective: work in industries associated with that element, live in directions associated with it, partner with people whose charts complement yours. Favor Wood? Try culture, education, publishing. Favor Water? Try trade, logistics, coastal cities.