First I heard of that Andy. I thought they were farm bred, like Trochus snails?
I purchased mine from Jayson @ SaltwaterConnection. I know that AJ also sells them or did, @ Aquarium Pros
Remember to feed your nassarius snails to keep them happy. They will eat crap and turn the sandbed, but need meaty food to survive. Look like monsters from the deep coming out at feeding time.
If one feeds a lot, then direct feeding is not needed as much. I use to just throw an extra bunch of mysis or brine shrimp in. They do climb on the glass also and sometimes into overflows, etc.
The conches dont seem to eat tank food, but crap on the sandbed surface. They love algae on the sand, esp red slime.
I have never seen a conch anywhere but on the sandbed itself, or under it.