I have no information as to where it is but I am guessing Warsaw as this is where my Dad was born
The fact that your Dad was born in Warsaw does not mean his parents' grave is in Warsaw.
For Warsaw, there is a registry for the three main city cemeteries (Cmentarz Północny, Południowy, Wojskowy) which will give you the date of birth and the date of death and the exact place when that person is buried. However, some other important cemeteries (for example Cm. Bródnowski, Cm. Powązkowski) are excluded. I have just found my grand parents' grave using their search engine! Click the "wyszukaj grób" and then put in the surname or date of birth or date of death.