You'd think there would be a source for new springs but I didn't notice anything for part # 66601. I've purchased quite a few similar for the Singer potted motors. I believe 188k and 306 use the same spring so you might find a good deal on a used bobbin winder--though this is not an ideal solution. Perhaps someone with a better parts source knows where one is available or check with the old time sew-n-vac or traveling repair man. Good Luck.
Jon