Sorry I don't know the answer to your question, but could you call the
WalMart that you plan to go to and see if they have it? If they have it maybe they could even put it aside for you if there's any concern that they might be out of it by the time you arrive?
When my youngest was on formula, he was on a special formula that was not always available in stores. It was made by Enfamil, so I called Enfamil and ordered it directly through them and had it shipped to the resort. Could you do that through Similac's website if Walmart doesn't have what you need?
If my DS had been on a more readily available formula, I would have ordered it from gardengrocer.com since we use them for diapers, wipes, water, snacks, breakfast foods, etc.
Good luck and have a fun trip with your little one!
