Order: Anaspidaea
They are herbivorous nudibranches most growing way to large for our tanks.
There are some smaller species available from time to time that are perfectly fine for our purposes. Excellent algae consumers that fish and other critters will not bother.
I can't help you on where you can purchase one though.
I love my sea hare! Great algae eater and so fun to watch. Not to mention they're so ugly they're cute. When there isn't much algae for it to eat we have been feeding those algae wafers that you buy for plecos and such and it will eat those. We've had ours for about 4 months now. So far so good.
The only critter that I've heard of that reliably eats Bryopsis is a lettuce nudi. Problem with those is they're rather expensive considering they usually last about 30 minutes in most tanks. They tend to get sucked into overflows and such. If you can keep them alive for long enough, they can help.
Bryopsis is a toughie. Aggressive plucking, accompanied by increased water changes, blowing detritus off of the rocks, scrubbing affected areas and a lit fuge stocked with macros is the best way to be rid of it I think. No fish eats it unfortunately.
Yeah, Ive tried all of those suggestions BUT it doesnt go away, (kinda like herpes of the aquarium). Im gonna have to break down and try a "natural" predator.