From Wikepedia, "One of the world's primary sources for caffeine is the coffee bean.....".
http://en.wikipedia.org/wiki/Caffeine
The primary source of caffeine is found in various plants, particularly coffee beans, tea leaves, cacao beans (chocolate), and kola nuts. Caffeine acts as a natural pesticide that helps these plants repel insects. In the case of coffee, the beans are roasted and ground to prepare the popular beverage. Tea leaves are processed and brewed to create tea. Cacao beans undergo a complex process to produce chocolate. Kola nuts, on the other hand, are often used in the production of soft drinks. If you are having trouble finding sources of caffeine, you may want to check local supermarkets, coffee shops, tea shops, or specialty stores. Alternatively, you can also purchase caffeine products online from various retailers.
