With some exceptions, the 9mm revolver utilize moon clips. These pieces of metal hold five rounds in a circular design, making dropping ammo and reloading pretty easy. Moon clips work exceptionally well and tend to be fairly cheap. The downside is that moon clips are flimsy, bend easily, and are kind of bulky.
Mags vs. Clips in the 9mm Revolver
Mags vs. Clips in the 9mm Revolver