I need some help with bearings and freecoaster

Ceramic bearings are a lot more expensive. You can get sealed bearings really cheap if you look around. If the cover has a code on it, you can use that code to find replacements. If it doesn't have a code you can measure the width, inside diameter of the inner race and outside diameter of the outer race and use those measurements to find replacements. You have to be accurate with your measurements though as some are imperial, most are metric.

As for switching your current hub to a cassette, it depends which hub you've got. Loads of people are after freecoasters so the easiest way would be to swap your wheel with someone.
Although, if you think you might want to switch between cassette and freecoaster, you should look into the ezra freecoaster hub, or salt plus trapez freecoaster hub. They can both be switched pretty easily and the salt one comes with everything you need to do it