janome machines use Class 15 bobbins. (class L will work, but 15s are what come with them.)
if you like the idea of pre-wound bobbins, i can recommend this site. they sell both polyester and 100% cotton threaded bobbins. the cost of prewound from this site is pretty close to the cost of buying empty bobbins and thread seperately. i've used dozens of them with no issues. they are "real" bobbins, so once emptied, they are reusable.
http://www.terradonembroidery.com/clear_quilt_15.htm