I am pretty sure there is no crafting recipe for Regenerate spell. You can sometimes but them at the Bazaar, or at Wizard101central.com in the card trading forum. I do not know which if any boss drops them. There are a few plants you can garden that may drop them; White laugh o dils maybe. They may or may not be available in a card pack like Bartleby's pack.
I too would be curious to know of more specific ways to obtain them.
