It may appear challenging at first but actually it is not that hard.
A good map helps. Something like this one is enough:
I can already trace my position on the map while in the bus. It is easy because there are many traffic jams in Colombo.
I bought this atlas in the University bookshop (Colombo 7) but I am sure it can be found in many other places.
Colombo bookshops are good. You can find many interesting books there.