This is the first thread I have ever seen entitled "assassin snails". I was thinking about making one myself.
I've seen deals on AB like 1 AS for $1 + $15 shipping. Quite abit more expensive, but maybe not that bad. Keep in mind, I do believe they burrow, so if you get 1, you might not see it alot. I don't recommend you put it with any other type of snail, but they are fine to live with there own kind from all accounts I have heard.
I think my lfs is looking into ordering 1 or 2 for me. I am createing a controlled infestation in 1 of my smaller tanks. I have dozens of ramshorn nest ALL OVER my java fern! SO many! Countless! Between my GS and my bigger snails, I am probably not feeding them as much as they need, though. I'll increase there food. I am working on Generation 3! Gen 2 is all grown up. Assassin snails favorite food is ramshorn snail. Gen 3 grows kinda big, I'll definately look closer into getting an assassin snail, maybe 2 if it gets out of hand lol.
Good luck! It is a beautiful snail!