Soldering flux and other hardware stuff in finland

I want to start soldering better, and build my own quad. For this I obviously need hardware like those tiny little screws, and soldering flux.

The hardware I can probably find and get online, but Im so confused on the soldering flux part. Like whats a good flux?? Where can I get flux? Apparently the tin already has flux, but thats not really enough... My iron is super oxidized and the tin wont even stick to it at all.

Im assuming a good clump of flux and one of those iron tip cleaners would do wonders but where the hell can I get good solder for a fair price??

For example I could get TBS solder from drone-fpv-racer.com, but that costs a lot for how much there is (its only like 10ml???). Then there is different kinds of flux in paste, in tubes, in like eye dropper looking containers, and theres lead no clean solder and leaded solder etc.

Whats the one that everyone uses for fpv stuff?