The closest shop is the internet. It is in your home. I buy tubes from
www.pricepoint.com, $2 each in quantity.
+1 I did not realize Walmart knew presta valves existed. Wild!
FWIW: Walmart's pricing on tires, tubes and other bike stuff is on the high side for a store that typically undercuts everyone else. I routinely buy cables, tubes, and tires on the internet for half of the Walmart price.