Hey, Doc. I've never stayed at POR, but have stayed at Disney quite often, and here's what I've learned over the years:
Yes, you can buy milk and such at just about every resort, but it'll cost ya! I think you'd be better of stopping at a grocery store on the way in from the airport and picking up supplies, and then paying the extra fee the resorts generally charge in order to get a fridge in your room.
I've never had a car at Disney, and have always relied on the Disney transportation system. I've never had a problem, but then again, I'm a very patient person. I don't mind tacking on the extra time to get from here to there. For many others, this is an issue (especially if they make a lot of priority seating arrangements or early character breakfasts, and need to get to a certain place by a very specific, early hour in the morning).
If you decide to rent a car, stopping at a grocery store won't be a problem. If you don't, you could always hire a car to take you from the airport to the resort, and most will throw in a brief stop at a grocery store for free (check the transportation board for more info on that). I don't remember ever seeing a foam cooler being sold on Disney property (not that I was specifically looking, but...). I'd guess they probably don't so as to encourage that you pay more for individual meals, but that's just a guess on my part. And again, for about $10 a day, I believe, you can get a fridge in your room.
Hope this helps! I'll be at POR from Nov. 29-Dec. 6, my first stay at that particular resort, but if you'd like to PM me when I return, I'll be glad to tell you anything I found out about the resort, food options and the stores on property!
Richard