If you can't get tamarind, you can also try adding a few drops of lemon or lime juice AFTER the sambar is done cooking (lemon and lime juice flavors change when heated). Another great Indian ingredient substitution would be green raw mango fresh in pieces added or dried added as a powder. In Goa, they would add kokum which is a dried mangosteen/plum fruit variety. Kokum is sour but also adds a purple-pink hue to the dish.