It is usually available at Kragen, Checker or Autozone. Pep Boys and Walmart DO NOT carry the 0w-40. If you cannot find it, the 5w-40 Truck & SUV Mobil one is perfectly good oil. You may also want to consider the new Mobil 1 10w-40 EP if you can find it.
My local Walmart here in SC supposedly carries it...there are spaces marked for 0w40 on the shelf but I have never actually seen it in stock in the quart or by the 5qt containers...this is however the crappiest Walmart I have ever been in and thats saying a lot...Advanced Auto here stocks it but does not have it by the case, by the 5qt containers, and they never seem to stock more than 5qts at a time which makes you go to multiple stores to fill the 8qts or so of a Land Cruiser
I've never found M1 0W40 in WalMart (almost every other viscosity) but it is in Checker/Shucks/Kragen/et al, and also at AutoZone. HTH. One thing I've never found though anywhere is the 0W40 in the 5quart size, I'd love to get the stuff that way.