vtkPolyData inherits FindCell() methods from vtkPointSet (which is an implementation of the pure virtual FindCell() defined in vtkDataSet - look in vtkDataSet for documentation about FindCell()). Using vtkPolyData::FindCell() will work for most applications and is reasonably fast.
However note that this is potentially a complex operation as there are different ways to “find cells” depending on your application. If you really want to hurt your brain, look into the vtkFindCellStrategy class and derived classes.