Most likely it just needs a common 15 class bobbin case, but for which one I need to see the machine. Is it a front loading or end loading machine? Straight stitch or zig zag? It'll be a 15 class 1 o'clock case either way, the difference is in the size of the needle slot in the bobbin case if it's a front loading zig zag machine. Here's where you can order one in the US:
https://shop.sew-classic.com/Bobbin-Cases-Parts_c23.htm
Cari
BTW, the serial number on a Japanese machine is of no use as there are no records to search.