I am speaking from experience when I say DO NOT rely on Wal Mart for tires if you are running 27 by 1 1/4 or 700. Wal Mart tires were unbelievably bad, with 26 inch tires stamped 27 inch, and other tires that wore out so fast and actually bubbled in places after three hundred miles. Estimate your route, and know where bike shops are along the way. You will get better quality gear in bike shops.
Do not get patch kits in Wal Mart either. Get all the patches and glue you could conceivably need before the trip, and carry that with you; get them at bike stores, or for patches, somethines you can get kits at automotive stores. Especially on the southern tier, you might be surprised to see how far you can travel without finding any stores that have anything to do with cycling at all. Carry at least one new spare touring tire of high quality, and a couple of spare tubes. If you see on a map that you will be traversing hundreds of miles of small towns and desert land, stock up on extra tubes.