Here is a GREAT video of them actually eating the dreaded aiptasia anemone!
CLICK HERE to watch the carnage!
Here is the cheapest place online I can find Berghia Nudibranch (Juvenile).. and they are ON SALE now for $18.99 at reefscavengers.com
Hmmmm .. here is another place that sells them... under "MISC>>"
http://www.aquatictech.com/inverts.htm
"Aiptasia Eating Nudibranch
Aedidella stephanieae
formerly Berghia verrucicornis
sexually mature adults $29.99 *
juveniles $20.00 *"