What color was it and what kind of fit was it? You should be able to find a similar leather pant in any fashion store. You can specifically ask the store owner what you want and they will let you choose a similar style.
Cameron Diaz does not have her own clothing line or brand as far as I know. Where did you see this picture? If you can let me know the event and date, i can check out what you want and point you to a store that sells similar stuff.