The tamarind tree, is indigenous to tropical Africa and has since made it’s way around the world, being particularly prevalent in South Asia, the Middle East, and Mexico. In fact, India is currently its largest producer.
The tamarind tree produces pod-like fruit. Open up one of these pods and you’ll find a few large inedible seeds, incased in a thick and sticky pulp. The pulp is what gets used and it’s flavor is tart, slightly sweet, and gives dishes a distinctive flavor. It’s totally unique and therefore makes finding a suitable substitute difficult if not entirely impossible. If you have Asian, Indian, or Latin grocery store nearby, check there. I’ve also seen it at some of those farmers market-type stores. Otherwise go online, you’ll certainly find it there.