You can buy replacement shuttles and shuttle springs on eBay.
When the shuttle springs get worn on these, they can't apply tension to the bobbin thread any more - replace the spring and you're back in business.
Be careful when buying modern repro bobbins for this type of shuttle. I haven't found a modern repro yet that's long enough to fill properly when you put it on the bobbin winder. You will need to guide the thread by hand in order to fill the bobbins. The bobbin winders are made for a specific length of bobbin - when you try to wind the modern repros on them, you end up with a dog-bone effect. :)
They sew just great - filling them is kind of a pain. :)