There are two -
Misa Travel
http://www.airfares.com.sg
Reliance Travel
http://www.reliance.com.sg
If you are in SIN, the best place to go on a search for the best rate from travel agents is in Chinatown (outram park is the nearest station, or chinatown station is station if it's been opened yet..), go to the basement of the Furama shopping centre, there must be at least 20 travel agents down there to choose from.