They are available where you rent the boats at Port. Althou it was free we did however have to provide a credit card to get it.
While at fort wilderness the lifejackets were just at the lifegaurd towers for day use.
You can get them from the marina where you rent a boat or bike. You will have to sign a paper and give your room number. If you check out www.portorleans.org., there is a thread in the discussion forum and I posted pics of my kids in the life jackets.