actually u can find sunbirds practically everywhere.....
the olive backed sunbirds r very common..... u can just go to any park...... look for any trees or bushes which bear flowers..... wait and u should be able to spot them feeding..... cheers.... (patience is the key to success )
Best place for shooting sunbirds can be found at the Heliconia Walk (near to the Symphony Stage - when u face the stage, the walk is on the upper right-hand side). Can't miss it as there are lots of Heliconia plants there. The sunbirds usually come down from nearby trees to sip the nectar from the flowers of the plants. Can get fairly near them, sometimes more than enough to fill the frame with a 300mm lens.
But as flaemer said, the sunbirds can be found at almost any park which has flowering bushes/plants or heliconias. Just be patient and wait for them.