I often hear others say that watching TV is Haram. My question is, for drama in which men and women all are in normal clothes, meaning they are not naked, nor are there any nude scenes, and with a great story, is watching this type of drama also Haram? are there any authentic/reliable sources which supports it is Haram?
Asking "Is watching Drama haraam?", is like asking "Is a pencil haraam?" where both questions can't be answered unless the questions are given further direction, as in their current state, are too general and can't be answered with any useful meaning.
We know that we have been told to lower our gaze. The prophet of Islam would lower his gaze when women would pass in camel-borne sedan chairs, i.e. he would lower his gaze even when he could hardly see the women as they were covered in sedan chairs:
Here is an example of what sedan chairs probably looked like in those days considering the required cover from the burning desert sun:
We also know that both men and women have been told to lower their gaze:
So, it doesn't really seem to matter if they are wearing "normal cloths" or revealing cloths, it seems you should be lowering your gaze no matter how well covered the opposite gender is.
Therefore, if you're male and the drama you watch does not have females, and if you're female and the drama you watch has no males, then it "might" be all good. However, all I can really say is, watch at your own risk.