Gambooge: Although the traditional requirement is gambooge this can be substituted for 1/8 cup of tamarind. I haven't seen gambooge in Indian stores but you can get this from kerala stores or if you know someone from kerala I hope they would be happy to share it with you.
If there is absolutely no way of finding curry leaves, you can add cilantro leaves before serving.
The longer the curry stays the better the flavor is absorbed into the fish. Let it stand for 2-3 hours before serving and serve this at room temperature.
Using skinless firm fish steaks like salmon, king fish, mahi mahi are ideal for this curry. Moreover you don't have to worry about cleaning.
Cleaning the fish: 1st remove the scales by moving a knife vertically on the fish. Cut the fins to remove any slimy portions. Now rub salt and fresh lime juice generously on the fish. Wash and drain.
I use mild chilly powder. So adjust it as per your preference. If the curry turns out too spicy, you can add some coconut milk, just bring it to a simmer and remove the pan from the heat immediately.