I've never seen them either. My guess is that they aren't easy to culture in commercial amounts.I've always kept my eye out for anyone selling freshwater copepods
I culture <"Crangonyx pseudogracilis & Asellus aquaticus">, they are easy to keep and smaller than <"Hyalella azteca">.Other than maybe scuds
I found when I was sending people moss they were subsequently getting Asellus in their tanks , and I've found most of these small crustaceans travel pretty well in damp moss.I'll have to remember to drop you a message. Sounds interesting.
We used to deal with Blades at work, and a couple of times they sent us something close to what we had ordered, but not exactly right. I contacted them mainly to tell them that their cultures were contaminated, and they replied by telling me that they didn't culture most of their organisms and that the ones they had sent us were "wild collected" and somewhere on their website there is a disclaimer saying you might not get what you ordered, so it was basically "tough luck". So I no longer deal with them.