I always stop at Wholesale.
They have flats of Remington Gun Clubs or Federal Top Guns for around $45. They probably only have 7 1/2's left this time of year. They seem to run out of 8's and 9's very early on in the season, and never bring more in. What you seee is what you get. They don't keep any of the flats in storage in the back.
You should be able to get them at Phoenix for around the same price. I don't know about P&D, as they may have to be a little higher because of the shipping costs for a smaller buisness. It's definately worth a phone call though.
Walmart has the promo packs of winchester or Federals for $20 for 4 boxes. They do in a pinch.
If you are shooting at a club, they may have shells to sell you at a reasonable price. I know ours does.
The best thing to do is try to get in on an order through your club in the early fall. If you are buying any quantity,(like 5000 shells +) it's worth the few bucks a flat to get in on a bulk buy.
Bill