Yes, most Walmart stores do sell US postage stamps at their stores.
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 about $8.00. 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 at Stamps.com and get FREE postage!