i bought pre-wound Class 15 bobbins from this site:
http://www.terradonembroidery.com/clear_quilt_15.htm
they sell both polyester and 100% cotton
having plowed through several tubes of each i feel safe vouching for them.
they use normal bobbins, which means they can be used again and again for very close to the price of empty bobbins plus [reasonably priced] thread.