do you know where to buy(buy) a washing machine?
washing machine 清洗机
where等疑问词后接不定式to 是一种结构
例如：I don`t know how to choose them.
I cannot decide where to go.
疑问词what, who, which, when, where, how和连接词whether之后,可以跟带to的不定式,构成一个特殊的动词不定式短语,可以在句中作主语,宾语和表语.
如：How to solve the problem is very important.如何解决这个问题是很重要的.（主语）
He didn't know what to say. 他不知道该怎么说.（宾语）
The question is where to find the source of water.问题是在什么地方能找到水源.(表语)