"To buy something for someone" 用英语可以说成 "to get/purchase something for someone" 。
来源：这是一种常用的、基本的英语表达。
例句：
Can you please get me a coffee while you're out? (请你出门时给我买一杯咖啡好吗？)
I'm going to purchase a gift for my sister's birthday. (我打算为我姐姐的生日买一个礼物)
I got a new book for you as a surprise. (我作为惊喜给你买了一本新书)