Hi There... you can try the Zoom hebdo which is a new paper released on Friday which has housing and rental ads.
Or if you are in Libreville just drive around Batterie IV you will see quite a few signs for rent. Batterie IV is a good area to live.
Or on facebook look for rental in Libreville
Try the website : Gabon coin
Or rental agent in Gabon. There are so many so use the practique d Gabon - it is like a local directory ...you will find all kinds of information in it.
Hope this helps.
Regards
Jenna