Used bassinet mattress

B17
My husband and i got a bassinet from.his sister who has had the bassinet for 9 years. Two different kids used it. Im fine with using it but im worried about the mattress pad.. Do i need a new one? And is it like a one size fits all if i look on amazon?or do i need to know the bassinet style and brand in order to get a matress pad that fits it??? Thanks !!