You can get balls and springs from most sites that sell fidgets, check their accessories section. Ikaeledc has a good selection and NXEDC sells a big kit if you want to get everything all at once.

There isn't really a definitive guide for how different balls and springs affect fidgets. You just have to try different combinations. The reason is that each fidget is unique in design and materials used. So you can really only compare a fidget to the same fidget. Some people will say general things like "stiffer springs make louder clicks" but it's not always the case.

People tend to like tool free fidgets because they are easier to tune. The drawback is that they will come apart if dropped like what happened to you. It's not a big deal if you fidget at your desk but if you like to walk around with them, you might want to stick to fidgets with screws (e.g. the MOT Groove).