Looking for 1 week apartment Dortmund/Bochum/etc

Hi everyone!
My parents and grandma are visiting me in one week from 14th until 21st august and I need a place for them to sleep. I´ve been looking for hotels or local apartments and I find Protected content for one week kind of expensive for what this is!
I´m pretty sure there are free apartments for people who goes on vacation.. is August!
So, maybe someone can help me, i´m out of websites to look.. any ideas?
Thanks a loooot!!!

Alejandra