How do you mean 'come apart'? did the spring just come out, or did the whole thing fall apart?
If the spring just came out, it will go back in with a bit of fiddling. It only goes in one way. There are 2 prongs on opposite edges of the spring, one set is a bit wider apart than the other. It will only go back in the case one way.
I got a couple extra on ebay (look up bobbin case for tin lizzie). They were under $10 and work just fine. My dealer wanted $25 each.