You can also use nutritional yeast. I have found it is different grocery stores that specialize in organic/natural/alternative products. Where I live the stores are called Natural Groceries and Sprouts. I think Whole Foods might carry it too. It's in the area were they have the supplements. You can also buy it online.
I've never heard of pur-a-yeast so I don't know if that would work. I've got a container of nutrional yeast now for my ducklings. I'll take a picture of the label so you can compare the values. If there is anything extra the pur-a-yeast contains you'll want to make sure it's safe for ducks.