I have seen M1 5W-20 EP at Walmart's near me. My guess is that the store has to be a larger than average SuperCenter, and with demographics of above average income (where customers are more likely to have a newer car that specifies 5W-20 and are willing to pay more for EP).
But I admit that they do happen to be frequently out of stock of 5W-20 EP. I know that EP was under severe allocation until early this year because of Hurricane Ike (more so than most other M1 products) but don't know what the problem is now. It could be that it flies off the shelves, or that they don't restock very often, not sure which.