We used Merckens Green candy wafers, which can be purchased at cakesnthings.com. Originally we thought that using a leaf mold might work for lettuce (you can see some of them on the bottom layer of the salad), but we discovered this method that worked even better:
Melt 'em: You may find that one method works better for you than another. We melted them in the microwave, but many swear by the double-boiler method.
Spread out a long sheet of plastic wrap (12 inches long or so). On one half of the plastic wrap, spread a 1/3 inch thick layer of the melted confection.
Fold the unused portion of the plastic wrap over the candy, and smooth down to a desired thickness (not too thin or it will break!).
Wrinkle the still-warm candy gently with your fingers, to give little wrinkles and ripples like on lettuce leaves.
Let cool for 20-30 minutes or until solid.
Gently uncover. Pieces may break off at the ends, but this is ok--lettuce is abnormally shaped after all!
Garnish as desired (with cake cubes for "croutons", red cookies for tomatoes, etc)