Tamarind or tamarindo is commonly used in Mexican sweets and drinks. It has a strong tangy flavor, and needs plenty of sugar.
Look for unbroken tamarind pods that are not too dried out, nor that show any evidence of bugs. Crack the shells off and remove veins before boiling the fruit. Boil in water until the fruit separates easily from the seeds, about 20 minutes. Press the fruit through a wire sieve to collect the tamarindo puree.
Tamarind is used throughout the African and Asian continents as well, so markets catering to those ethnic groups should have a good supply.