Paper ball shells are the least expensive ready-made, spherical shells you can buy. They come as two halves. You fill them with burst and stars, fuse them, and then paste them with strips of paper or tape to complete them. See our Fireworks Projects for complete instructions on making this kind of aerial shell.
Cylindrical (also called canister) type shells usually consist of a "can" and a top or bottom that snaps in. Spherical (also called ball) shells consist of two hemispheres plus some accessory parts. The various sized shells come as complete sets. For instance, when you purchase PL2050, 3 inch spherical shells, you get a bag containing all the plastic parts to make 50 complete shells.
To assemble them, you should also have either methylene chloride or xylene to use for bonding the parts together. The Solvent Applicator is our personal favorite for doing this.