I was lucky enough to be able to rent a studio from an acquainted for most of my time. The studio was really fresh and very close to the LUMC (the university and hospital). However for the last two weeks I had to find somewhere else to live but I just wrote a post in a facebook group called housing in Leiden and got a lot of people responding to me. This house was further away from the university, but still pithing biking distance, and I shared it with five other people but it was still clean and a good environment.
I have heard from other people that it have been harder for them to find housing so I would suggest to start looking for it as early as possible. If you do not have anywhere to live the first week of school the OWL week offers you to sleep in a gym hall so you still have some time to look for an accommodation.
The university also offers to help with finding housing however this costs about 350€ and you are still not guaranteed a place to live so for me this was not worth it.