another source. they have both cotton and polyester; reasonable prices; class L and class 15 (best size for Janome). all are plastic. judging by the picture, you can easily re-use their class 15s. i'm not sure about the class Ls since they don't have the little holes in the tops.
http://www.terradonembroidery.com/clear_glide_15.htm
i've also used the prewound bobbins from quiltinaday.com, which are class L (no little holes). even though they aren't the "correct" size for my Janome, they work just fine for piecing and the low-bobbin sensor works.
i guess we could drill little holes in the used class Ls and re-use those, too.