At Ming Tsai's place, I heard about Kaffir lime leaves. I have seen more with his recipes on his TV show. I don't think I have seen him explain more about these enigmatic leaves which I can't find in the Kansas City area. But first I went looking for substitute ingredients. Here is what I found
(pic at google //yeehawmurghi.vox.com)
substitute frozen for fresh, or use lime zest from common Persian or Mexican Key limes
KAFFIR LIME LEAVES
There is no perfect substitute for this unique item. You can substitute lime zest for the citrus taste or use dried Thai sweet basil or even dried bay leaves for a woodsy-leaf flavor. Also try lemon thyme, lemon verbena, bergamot powder, lemon or lime leaves. Since kaffir lime leaves are difficult to find, do not let this keep you from preparing delicious Thai dishes. They are optional in many of our recipes. You can leave them out and have the other ingredients in your dish convey the unique Thai flavors.
(Also see Thai Ingredients.)
“A kaffir lime leaf look as if two glossy, dark green leaves were joined together end to end, forming a figure-eight pattern. Most Thai recipes count each double leaf as two separate leaves. Frozen kaffir lime leaves are a good substitute for fresh. Dried leaves are much less flavorful, so use twice as many as the recipe calls for if you're substituting them for fresh leaves. Substitutes: lime leaves OR kaffir lime (One tablespoon of zest from a kaffir lime is equivalent to about 6 kaffir lime leaves.) OR lime zest (One tablespoon of zest from a lime is equivalent to about 6 kaffir lime leaves.) OR lemon leaves OR lemons (One tablespoon of zest from a lemon is equivalent to about 6 kaffir lime leaves.)”