buy one of them / buy one
Pour aller à l'école, il faut en acheter un.
To go to school, you have to buy one.
— Pinocchio (2019)
I need to buy one.
Peux-tu en acheter un pour moi aussi ?
Can you buy one for me too?
I have to buy one tomorrow.
Where can I buy one?
Vous pensez que votre mari pourrait m'en acheter un aussi ?
Do you think your husband could buy me one too?
— In the Mood for Love (2000)
I have to buy one.