I'm not sure you would find it at Walmart. The Walton's frown on sodomy.
I have found it at the local adult supply store or you can order it online if your worried about discretion.
http://www.amazon.com/Pipedream-Products-PD9804-62-Anal-Desensitiz/dp/B007YSIL7U
Numerous partners were glad I had it on hand over the years. Did I just publicly admit that I'm thick?