Bilinbi and Sour green mangoes

                                        

Bilinbi
Resembles a small cucumber, but has a softer flesh with a sour taste.
Used as a souring agent in curries and also used for making pickles. Replace with small, green mangoes or lime juice if you don’t have any bilimbi

Sour green mangoes
These are young mangoes, so young that you can slice right through the seed. They are quite sour in taste and are used as a souring agent in curries and also used to make pickles.

If neither of them is available, tamarind or lime juice is used depending on the recipe.