From where to start learning new tactics? I know the basics and some basic tactics in chess, but I want to learn new and advanced tactics. Can anybody tell me any source for that?
I have tried some of the existing sites that provide exercises. 

If you are searching for common tactical position, chesstempo is sure one of the best options with ilimitated free exercises.

If you are searching for incredible checkmates, sacs etc, chess.com has a nice trainer, the problem is it is not free.

You have also lichess. The exercises at this site are taken from blitz games of the players. They use stockfish to generate them. Some of them are doubtful, but a punctuation system is doing his task. 

If you want to become a good player in long games (90+30) I recommend you to do lots of exercises at chesstempo, but lichess exercises are nice for tricks and so and totaly free.