I stopped to see the gentleman I mentioned. He had a big box of assorted all class 15 bobbin cases that we went through. I never realized how many different class 15 cases there are, some without the finger and that have the opening on the top. However, they threaded differently, the thread came up through the tension in the back, the standard bobbin threads through from the front. Another thing that you will need is one that fits over the standard pin, not all pins are the same size. I tried 2 different ones that I have here and neither fit the standard pin.
He did say that your best bet would be if you can come up with an old machine for parts.
Sorry I can't be more helpful.