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
To find USPS stamps at store locations near you, just type in your zip code here: http://www.uspseverywhere.com/