Your host university will generally help you to find accommodation. If you receive an application/registration form for accommodation with the other documents you are sent, be sure to return it to your host university in good time.
Support from fellow students
Contact students from Weimar already studying at your host university as soon as you have accepted your place. They should be able to advise on accommodation and potentially even to pass their accommodation on to you.
Why not sublet your accommodation while you’re away? It’s very simple (provided your landlord agrees): simply place an ad in English in the Accommodation section of the Piazza message board.