As of September 2015, Lowe's online site notes that it does not stock any form of flame retardant spray. The store does sell various other fire retardant materials, such as insulation foam and resistant boarding.
Not many retailers stock flame retardant spray, and sourcing it from local stores is tricky. A homemade retardant spray is made by mixing seven ounces of borax and three ounces of boric acid in two quarts of warm water. If the article is washable, soak it in the solution after rinsing, then air dry. If the article is not washable, spray it with the solution using a spray pump bottle. These are available at Lowe's. This solution washes out of clothing, so use it after each wash or dry cleaning.