Some mills have a planer. A belt sander may get you where you need to be as well. You are going to have to cut a lot of it anyway, so the outer surface condition probably isn't relavent
Be forwarned that wood that size will take at least a year in a warm, dry place before it stops moving around. If what you buy is not dry, you would be best to get it an inch or so oversize in every direction and wait a year, then cut it. It sorta sucks that it takes that long but it will (and that is an optimistic figure, 3-4 years would be better).
If it isn't dry, every time you put it in the sun the wood will bend towards it.
It would be worth calling around though. You never know what a mill might have hanging around.
Pine will rot quickly if left outside.
Home Depot wood, besides being the wrong size, will be in the neighborhood of 3-4X what mill wood will cost (at least that is my experience in my neck of the woods). When I told the guy who owns the lumber mill up the street I was buiding a mortar, he went way out of his way to help me out. I paid somewhere in the neighborhood of a little less than a dollar a board foot for white oak. I got pieces 3-4 inches thick, about 1/2 inch oversize in length and width, so I could clean it up and trim it all to the same width/length. It wasn't quite dry... I learned the hard way what that means. Even though I used several bottle of glue, and 6 hardwood pegs between each lamination, next year I will have to sand it down, fill the lamination separations with epoxy, and repaint it. A couple of 8 inch thick pieces would have been a nice find, but getting dry oak 8x10's would be a miraculous find I believe.