If you go around Kelaniya, you will see small notices advertising available boarding places, Specially near the Bus Halt of Kelaniya University. Also you can ask some info from Kelaniya University students. They are help full (my experience). Or ask a shop owner, some may help providing details or will show someone who knows about. but if you are dealing with a broker ask what is EXPECTED from you (eg one month rent as commission). Bless you get a good room soon.