Our toddler has a milk allergy that he has thankfully started to grow out of. However, for the last four years we have taken him to WDW. Our experience has been SOW milk was found easily at the moderate food courts (where we usually stay). But they are the small sizes and were last year about 2.99 (Silk brand) each (in the refrigerators in the food court and some of the gift shops actually).
In the parks we were able to find it at a couple quick service places as well, but I am afraid I don't remember the exact ones but I remember there were 2 or 3 in Magic Kingdom. We never had a problem finding it, but sometimes we had to walk to couple different places With our son we didn't really eat at the full service restaurants much. He was just to eager to explore for that.