Anyhoo, back to the pressing matters.
Don't worry about an agent. Go to Google and look for Lumphini Condo or Lumphini Ville.
Here is one in the area you are looking...
Google Maps
They normally have a few buildings in each complex with a notice board displayed at each entrance with condos for rent by the owner.
You can set up to view 5 or 6 rooms of your choice in the same afternoon and be out for beers before sundown.