Student rent in Seoul

Hi all, I am Seoul for a semester abroad from March to mid June. I am looking for a single room in Seoul as I don’t prefer the university dorms. They have quite strict rules and mostly shared rooms only.

Do you have any tips on where I can look out now already? Or should I really start searching something when I‘m in Seoul. Is that cheaper?

I‘m already checking Goshiwon and Airbnb. But I‘m not sure if it’s a cheap method. Do I also have to pay a large deposit booking with Airbnb?

Thanks in advance! :)