I don't know that a supermarket would sell one product that would contain everything a duckling needs.
You could try to find the online Storey's Guide to Raising Ducks. There is a chapter about mixing your own feed. Or see if you can find it at a library.
How about mail order?
By the way - do you mean for a very short time or long term?
Can you define what you mean by "supermarket", please? In these parts a supermarket is a grocery or big box store and I can't think of one good reason anyone would want to feed their ducks from the "supermarket" so I'm wondering if you're meaning something else. And, if not, what your motivation to do so is.
I can't think of anything. Chick starter might be medicated and not great, if you can get something like purina flock raiser its better. a round $16 here gets a 50lb bag and lb for lb thats cheaper than anything from the grocery store.