You are in luck, because they are listed in the lecture slides. In fact, the first four modes of a vibrating cantilevered beam are given in a table in the lecture.
You should definitely calculate a density based on the measurements that you took, and perhaps compare that with a 'standard' for stainless steel just to make sure what you calculated matches up with the standard within experimental uncertainty. You should be able to get densities like that from a textbook.