Areas around observatory are quite expensive. From your poster I guess you will most likely be studying at UCT. All the same, I will drop a link here for you, you can use it to secure accommodation. Just bear in mind that its going to cost you lots more than we pay back home in Nigeria. Concentrate on Mowbray areas, its a stone throw from Observatory, and Nigerians are quite a few there as well. I stay in Belhar which is about 20 - 30 mins from Observatory. Good luck
Link is
http://capetown.gumtree.co.za/