Yes, snails would do the job, and as a bonus the chickens love eating the actual snail, too, not just the shell, so it would be a great snack in addition to the calcium. However, are you going to be able to keep a steady permanent supply, for as long as your chickens are laying? You'll eventually run out of snails, or may decide it's too much work to keep up if that's your main source of calcium... Also, it will be hard to figure out if your chickens have had enough calcium. With something like eggshell, you know they eat it when they need calcium, and when they've had enough, they'll stop. With snails, they may keep wanting more and more because the meat is tasty, even after they've had enough calcium for the day/week. So you won't know when they legitimately just need more calcium. Sometimes you can find empty snail shells lying around, but that would be quite a bit of work to go hunting for empty shells regularly, and trying to find enough to meet your chickens' needs... It would be fine as an occasional supplement, but you'll need something more reliable as their main source of calcium.
Don't worry, it won't make her break her eggs. Oyster shell and the other store-bought supplements are a very new, very modern thing. For the rest of chickens' domestic history, farmers have been feeding them their eggshells for calcium. That's what they'd do naturally, too - a broody will eat the empty shells after her chicks hatch, so her body can get back lost resources, but then she doesn't go on to break and eat eggs after that. People worry about this a lot, but don't consider the fact that their broody hens do eat the shells and that doesn't become a problem. Egg breaking and eating is rare, and can happen whether or not you feed your hens eggshells. People who only use oyster shell also report the occasional egg-eater, who didn't get the idea from being fed eggshells. And the vast majority of people who exclusively feed eggshells don't report any egg eating among their chickens. If an egg gets accidentally broken in the nest, and the chickens see the tasty inside, that would make it a lot more likely for them to develop the bad habit, than if they only saw the shell. They seem to have a built-in safeguard when it comes to the shell - nature ensuring that the mother hen who eats the shells of her hatchlings, doesn't go on to eat whole eggs as well. So don't turn down this excellent source of calcium, which is easy to get (free if you have chickens!), and which the hens will love to eat. Crush the shells to about the size of a fingernail, and it will be totally fine.