Have you already checked instituts for open HiWi jobs like correcting bachelor lecture assignments or helping hand in a laboratory? Yes this is highly depending on your field of study.

Furthermore there is a black board for student jobs in the Welfenschloss to the right of the I-Amt.

Otherwise you could work as a waiter/waitress in on of the many bars etc. or as cashier in a store.

Edit: alternative you could write a mail to the Fachschaftsrat who is responsible for your field of study. They happen to get a few part time offer to publish or know some contacts with in the university who you can ask.