These toys were popular back when I was in high school and I still play with mine today. I got sick of the spike dulling on my wood kendama so I reversed engineered my kendama so I can just print new spikes when mine dulls. Super easy print and a fun toy for kids making it a great gift for the holidays. I printed my ball as a single print but as you would expect the underside of the ball warped a little bit so I also added files of the ball split in half so they can be printed separately and glued together to make a ball without warping. The one thing I would advise when printing a kendama is to get the weights right for a more natural and authentic feel when using it. I weighed my wood kendama so I could get close to the same weight using simplify 3D's mass estimate. The ball should weigh about 73g and the cup plus the stem should have a combined weight of 64g. Remember when you're using your slicers mass estimate feature it includes the mass of the support structure which I would suggest you use when printing the stem and cups.
The only thing you need to buy is the string but that can easily be made using a small bead and some sort of thin string. I will add an amazon link to a selection of kendama strings out there.