Heyyyy
Here is a nice website for an accomodation in Bonn :
click hereYou can apply for a student flat you have to explain that you are student from abroad with an UN internship, that you need a room from [date] to [date]. And you have to attach (e-mail) or send (mail) a copy of your student card and your internship agreement
the Postal adress :
Studentenwerk Bonn AöR
Hannelore Giersberg
Nassestraße 11
53113 Bonn
Email adress :
wohnen@studentenwerk-bonn.de You can use an automatic translator. I think that applying in english is ok.
Thanks Magali for the details and also Elise, Eva and others who spoke about this website