I Googled and found this place: 50 Class 15 metal bobbins are $15 (don't know about shipping costs):
http://sewingpartsonline.com/bobbins...l-10-pack.aspx
My bet is that jhittle would have the bobbins you want in large quantities for a good price. They are hard to search online, though. Try calling them at 1-877-805-2616