I use to have a Brother 1500 and the bobbins are the same, so I have many of those for my Juki.
Have you tried prewound magna glide bobbins? I have less thread issues and the bobbins last almost twice as long as the ones i wind myself. I wind color thread I may need for one time use.
I have bought Juki bobbins here:
10pk Metal Bobbins, L Style, Rotary #55623S