I just bought a box of 100 "fender washers" with a 20mm outside diameter from a hardware store for $6. They are steel and about a mm and a half thick. They have a small hole in the center (v. small, really - couple of mm).
These fender washers come in many sizes - you can get ones that are about 25mm easily, too; they were a little more expensive. They are nice and neat, give a pleasing weight to the model, inexpensive, and also magnetic.
Excellent things to base models on for AoS, especially if you are having a smaller base than usual (because the weigh makes the otherwise unstable models more stable).