Roland,
If no one expresses any interest in your sea hare and you don't want to take it to a LFS, you could probably feed it nori. I've fed it to some of my creatures and they liked it as well. Found this when I researched:
"i have had good luck feeding nori strips to my sea hare. it likes them. they do need to soak for a few minutes before it will eat them. they are to tough for it when it is just put in the tank. it has gotten used to me reaching in and grabbing it to put it on the nori magnet. it will also eat full stalks of sargassum."
An excellent place for nori is from any of the Asian stores in the strip district. It's extremely cheap compared to the "pet store" versions. I go to Lotus (the larger one which has most things labeled in English) frequently with my wife, just Google "lotus strip district pittsburgh".
Just go in through the doors, take an immediate left, walk straight past the sushi counter (on the right) and it will be about 6 - 10 feet down on the first aisle on the right (across from the baked goods display and to the left of the cookware & herbs aisles).
There are many, many kinds but the cheapest will work fine - it's basically all the same. Nori will come in sheets (like paper) and the packages are face up (like looking directly at a sheet of paper). Just make sure the only ingredient is "nori" or "seaweed" (ingredients are typically printed on the back in English) and that it doesn't have any preservatives (almost never does, but worth checking).
Good luck!
John