Yes, Walmart stores do sell US postage stamps at their stores. To buy stamps at Walmart just ask the clerk at check out for a book of postage stamps.
Walmart Supercenters stores that are open 24/7 should have stamps right at the cash register, so when you check out you can request a book of stamps from the clerk which should be $10.00. In the past the stamps were kept at the Customer Service center, so if the clerk does not have any stamps in his cash register, go to Customer Service to buy a book of stamps. Some Walmart stores have ATM machine dispencers that will sell 1st class stamps.
You can also buy stamps online for less at Stamps.com and get FREE postage!