Sita was likely born somewhere in jungles of Himalayas, perhaps close to Kailash mountain.
The story of Birth of Sita starts with that of Vedvati. When Ravan received strong boons from Shiva, he went around the earth fighting many Kings and destroying many kingdoms. Once he was roaming around in jungles of Himalayas, that's when he spotted beautiful Vedvati.
Sarga 17, Uttar Kand, Valmiki Ramayan
When Ravan insulted Vedvati, she decided to do agni samadhi. She deliberately didn't curse Ravan, instead wished to be born as Ayonija Kanya, or young girl born without a womb.
Sarga 17, Uttar Kand, Valmiki Ramayan
Vedvati indeed takes rebirth as a young girl like she wished for, lying inside a lotus. Thus, Sita was born. This place is likely somewhere in Himalayas.
Sarga 17, Uttar Kand, Valmiki Ramayan
The story does not end here though. Ravan took the girl to Lanka anyways. However, his mantri identified the girl as cause of Ravan's death, hence advised him to get rid of her.
Sarga 17, Uttar Kand, Valmiki Ramayan
Out of fear, Ravan threw the girl in Sea. Earth took her in her care. She reappeared again in the middle of Yagya bhumi of Raja Janak, where he was performing Yagya. (I believe this is the point of interest today).
Sarga 17, Uttar Kand, Valmiki Ramayan
Thus, Sita was reborn again. In fact, she is called Sita because she came out of line drawn by a plow. Unfortunately, nothing much is mentioned about the place of Yagya.
Sarga 17, Uttar Kand, Valmiki Ramayan
A less detailed but very similar story of How Raja Janak found Sita is told to Shri Ram in Sarga 66, Bal Kand, Valmiki Ramayan. Please see Shlokas 13 onwards.
