where to buy cheap things.是属于,特殊疑问词,加不定式的固定用法,做宾语成分.
特殊疑问词加不定式,是属于一类用法,比如
what to do
how to do it
where to do it?
当然,前面都要加上主句的部分
都可以和真正的宾语从句做替换
People will know where to buy cheap things
=People will know wherethey can buy cheap things