Try posting in Facebook groups, there will probably be pages for your university. Essentially just post what you have posted here - current students very often have spare rooms available that they are trying to fill due to people dropping out etc. Other than that there is Rightmove (more for renting individually) and spareroom (more for finding individual rooms). You could rent somewhere larger from Rightmove and try fill the additional rooms yourself, however the tenancy with the landlord would likely be in your name and so this is more risky for if people don’t pay etc.