knock sensor, first of all its not available in open market and if found with honda motors then wont be less then Rs12000.
your best chance is scrap yard but point remains that these sensors are bound to fail with time, if car is over all in good condition then its better you buy a new one.
get it directly imported from online networks as that is the best chance that you will get if for far lesser price.
good to know least you found the problem as i am still struggling with 2nz-fe engine " Question on timing Advance"
saw your thread and it reminds me, that these days, i am thinking maybe knock sensor is not working thats why i am facing these problems.