David,
If I understand the origin of this thread properly, any discussion of T. crispada (lettuce leaf, Brysopsis-eating) "nudibranchs" being of any use is a moot point.
We got the brypsis-eating "nudibranchs" (Tridachia crispata) that we gave you and Robie at Carolina Biological. With shipping they came to a little under $8. I had a little difficulty finding someone there who knew what I was talking about until I found a Marine Biologist there who is fantastic. If you ever need to talk to someone there, I can give you her information.
The sea slugs (I don't think that they are nudibranchs) worked for us with the Bryopsis (along with plucking with tweezers), but did not show any interest in other algae. That is why we gave them away. Our Bryopsis situation was cleared up and we didn't want them to starve.