i have been using generic class 15 bobbins - including prewounds - from a few different online sources for years. i have experienced absolutely no difference in performance between those and the ones that were included with my machines when i bought them.
the generics prewounds cost approximately the same as empty Janome brand. they have "MILES" more thread on them, too. to add to my savings, i buy only those that are reusable. i now have enough bobbins to last about 3 lifetimes. :lol:
for those in the market i recommend these prewounds:
http://www.terradonembroidery.com/clear_quilt_15.htm