Available at most game stores, Twilight Creations puts out roughly 25mm minis for their Zombies! games for super cheap, about $10 for one hundred in a bag. I would recommend that any GM should have a Bag O' Zombies, a Bag O' Babes (female zombies), and Bag O' Skeletons (they are equipped with spear and shield), and a Bag O' Dogs (zombie dogs). Some come in glow-in-the-dark form. If they seem too small, just glue em to pennies or some other bases to give them an additional (scaled) six or more inches of height (it also makes them a bit more stable).
Twilight Creations @ Amazon