* Plain has a homophone plane, which refers to a flat surface or an aircraft.
* Witch has a homophone which, a pronoun used to ask questions.
* Flu has a homophone flew, the past tense of "fly".
* Bye has a homophone buy, which means to purchase something.
* Was doesn't have a common homophone, though it can be confused with "wasn't" in certain contexts.
So, the homophones for your list are: plane, which, flew, buy.