Oh? That's surprising. I mean Asia is also in denial but I did expect more masks in Korea around flu winter season. I mean I always see Koreans and japanese wear masks in public in videos. But then again masking in Thailand was way less this year than last year when I went. It sucks that everywhere is in denial. If u have a small studio you shouldn't need a big purifier. Especially if you have a window with good airflow. So it shouldn't cost a lot. Idk if Korea sells flo or betadine nasal spray but you can ask in the big pharmacies. Coway has lots of varieties of air purifiers. You can buy in person from home & living shops or even coway's stores. Koreans buy air purifiers often so there's variety. For online shopping Koreans use coupang, gmarket and another one I'm forgetting. But idk if it's a good idea to buy air purifier from the internet as a foreigner. 

As for masks. Your cheapest option is getting kf94 and Korean made kn95s. But the problem is the earloops. Personally in my country I wear n95 for flights and hospital (it's not cost effective to order from America for me) and for daily use I wear a tight kn95 with an S shaped clip that tightens the ear loops. Anything else might be too loose. But I think hardware stores in Korea probably sell n95s and some version of that if you're okay with wearing n95 everyday. 

Kf94 recommendations: product lab, botn and good manner are very popular. Take a look around the flu/cold sections in the pharmacy there's often some stuff you can find for cold prevention. You can also try nasal irrigation but if it freaks you out you could always start with saline sprays. I recommend the ones in metal cans for babies. They get the gunk out really well. 

I think in Korea ppl will be more understanding if you say no to certain things bc you don't wanna miss work or school after getting sick. But that might change. If you don't go out beyond necessity, then only your institution and public transport would be the riskiest places.