Hey Piter,
This is a type of fruit which is used for cooking curry or soup in Indian culinary. Taste extreamly sour. Sorry, I don't know the name for it and it seems to grow in warm area. To get the seeds, one has to find a ripe fruit which can only get from the tree itself, those fruits selling in the market are young one which the seeds inside are still too young. It's hard to find such tree in here, sometimes I did see it but can't recall where.
If you can wait, I'll see if I can find it.
Why you want this tree for?