Reef cleaners is a good source but I have to agree about the sizes being quite small. I'm also never really sure about how many live. The part I really don't like are that they include Cerith snails, which tend to fall off things, land on their backs, and die because they usually can't flip themselves over if they fall on sand. Instead, I really like Trochus snails. They're a little bigger and cost a little more but can flip themselves over and best of all, will breed in the tank. I started with just 5 a year ago and now have at least 12.
Be careful buying too many. Reef cleaners tells you that you need scores of these things, and if you feed heavy or have a large bio load, maybe you do, but if not they clean up the tank then starve and have to then clean up each other. Then all the stuff they cleaned up goes back into the water and you're left with a substrate of Coraline algae covered shells.
The other thing to watch out for is that a lot of cheap on line stores source their snails from (relatively) cold water, like the Carolinas. These snails will live for some time in a reef tank but the higher temps increase their matabolism and drastically shorten their lives.
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