Shrapnel:
It's possible that BlacKat has his/her terminology mixed up.

A lot of people use the term "bullet" when they mean "cartridge", and
vice-versa.
BlacKat:
Try your local military surplus store--while they rarely (if ever) sell *live* ammunition, it is possible to find dummy or deactivated rounds--the difference being that that dummy ammo is inert (incapable of being fired at all; it's primarily used for display or weapon-familiarization purposes), and deactivated ammunition was once live (capable of being fired), but is no longer. The usual method of deactivation is by removing the gunpowder and/or deactivating the primer.
If you're looking for a cartridge for display/demonstration purposes, I'd suggest looking at a website that sells cartridge-based jewelry--keychains, necklaces, and the like. An example that comes to mind would be the Lone Ranger commemorative cartridges, which are nickel-plated .45 Long Colt cases with Sterling silver bullets. They contain no gunpowder, and there is a piece of metal
--either a small loop in the primer pocket in the cartridge case's base--a small loop in the case of the necklace, and a spring-loaded key ring in the case of the keychain version.
There are also places that sell inert ammunition for collector purposes--some people collect cartridges; others use them as props for display purposes. The latter are usually used to fill the cartridge loops of gunbelts and the like when the weapon isn't intended to be fired. For example, the cartridges frequently seen in the old cowboy pictures--and more recently in the ammo belts of machine guns in war films--are props only, as the weapons are modified to fire blank cartridges.
Another possibility would be a sporting-goods store--they frequently sell "snap caps", which are plastic dummy cartridges of the same size and shape as the live ammunition of the same caliber. The idea is to allow "dry firing"; that is, operating the weapon's mechanism safely--say for indoor shooting practice--without the risk of an accidental discharge. Such devices are also used for that purpose to prevent possible damage to the weapon's mechanism, as repeated dry-firing can cause the firing pin or spring to be damaged.
If all you want to do is illustrate the size of a given cartridge, any of the three would be good choices.
--Foreigner