There is no such thing as a Stupid question when it comes to quilting. I made this same mistake recently. My machine came with 4 bobbins that I haven't needed more. Used these same bobbins for the last 6 years.
Saw some on sale at Wally world and bought them. They are COMPLETELY different than my machine.
I have a Janome 415 and it takes specific Janome bobbins. Not all bobbins are created equally as there are many different shapes and proportions to the individual bobbin casing per type of machine. JoAnn's had a very good sale on bobbins recently and I found out their New Home line of generic machine parts and accessories work very well for my machine. I would encourage you to take your owner's manual to your favorite store and talk to a knowledgeable clerk. I don't know if there is a cheaper version for your machine, but there may be and I would start there. JoAnn's or Hancocks first. Somewhere where someone knows what they're talking about, but not bound by dealership stuff to make you buy really pricey stuff.
Hope this helps.
Melissa