I've never had a zombie attack my Sim, they were always much more interested in eating the garden. If your Sim turns into a zombie, hit them with a Cure potion or a Potent Cure.
The stock for the Elixir store is random, and only ever stocks a few prepared potions at a time. Usually only 1 or 2 recipes. And mostly components for making your own elixirs. Alchemy is your friend. Otherwise, try buying what's there and seeing if it restocks with something else. I once spent over 200k trying to find a specific potion for sale there. And as far as I know, every potion that's for sale can also be made at the Alchemy station.
