I picked up some 8MM Steel balls on amazon for about 10$ Here's the exact ones I used. Just so I can make some smaller ones, for less money. (Instead of 4$ for one spinner, it's only about $2.50 for one.) The end caps in the picture aren't actually included in the STL, they're the same ones that I uploaded a week ago (Found here). The fidget is a tight fit, if you want to get it out I'd recommend a blowdrier or some heatgun, and to get it I simply put it on my counter top and hammered it in. Hope you have fun with this tiny design!