Quote from CET:
You should check out Theanine, which is found in tea. It improves focus and provides a calming effect, which offsets the jittery effect that some get from caffeine. You can get Theanine as a supplement if you don't want to drink tea. If you choose to drink tea I believe green tea is the best choice, as it has many health benefits.
If you want to buy Theanine I suggest you check out Vitacost, as they have good prices. Their NSI brand is usually the best deal. You should be able to get Theanine at most health supplement sites. FWIW, it will probably be labeled as L-Theanine on most sites.