I use plastic bobbins all the time. I have a Singer and I had to get a certain size (they are not universal). If you Google Janome plastic bobbins, you will see that there are bobbins to fit. That is what I had to do with the Singer. Now I am perfectly happy with my plastic bobbins - They come in packets of 10 and aren't that expensive. Then I just load them up. Also, you can check with JoAnn Fab and see if they carry the size for the Janome. That was where I got mine. Good luck to you and happy quilting! Also, use those 40-50% off coupons for things like your bobbins. Then I just double up.
Edie