I managed to find some at a local LFS. He said I was the only person who knew what they were LOL. Years ago I got some from IPSF and had been trying to find some this time around. If you have a local reef group where you are, you can check around & see if anyone has some, or you can see if your LFS can order some. I started with 5 in my fuge in I think March, then when I moved a turbo from the fuge to the display it had eggs on it. I have too many to count now LOL. We've been giving them away to local reefers in our co-op.
I've never seen collonistas or rissoids for sale, but you usually end up with them from frags.
Stomatella snails are great too, and will breed in your tank. I found some at an LFS that weren't for sale & talked the guy into letting me buy 2. They bred like crazy but my original cleaner shrimp ate them all (along with some snails -- not really normal for a cleaner, IMO), so I only have them in the fuge now. Usually you'll get those with frags too, or from other reefers in your area.