Since you're going to Walmart, look for the following in this order:
RCA ANT751R (not in all stores)
Antennas Direct ClearStream 2V (comes with 20" outdoor mount, purchase coax separately).
Ignore all the little flat and indoor antennas. You need something larger and you need to put it outside for best results..
Mount the antenna outside, up in the air so that it has the clearest possible view in its line of sight, due south.