Regular lime leaves as well as lime zest can be substituted for kaffir lime leaves. Lemon or lemon leaves are also suitable. Kaffir lime leaves are often used in Thai cooking and are available fresh, dried or frozen.
To zest a lime as a substitute, wash and dry the lime. Hold the lime still, and run a zest grater over the lime. Only take the green part of lime, as the white pith can be bitter. Scrape the zest into a measuring spoon to measure the amount. A tablespoon of lime zest is usually enough if the recipe calls for six lime leaves.