I don't see it in my plant list, and I have checked over and over again. It's not there. What is the sun cost for it?
First aid isn't a separate plant, it's the ability to replace damaged blocking plants (the question is for PvZ2, but it seems the behaviour is the same as in the first game).
Wall-nut First Aid allows the player to plant new Wall-nuts, Tall-nuts and Pumpkins on top of damaged plants of the same kind without having to dig them up first.