For the fish:
You can use any fish that you prefer. Mackerel, pomfret and tilapia would be good. You can even give salmon a try!
Tamarind:
For the tamarind juice, you can either use pre-made tamarind juice or concentrate from the store or make your own juice. Take about 1 tbsp off a tamarind block and soak it in ¼ cup of water. Squeeze and strain the juice and this is ready to be used.