You can purchase a City of St. Louis flag at
"The Flag Loft" 1900 Delmar Blvd. St. Louis, MO 63103 Phone: 314/621-5417 FAX: 314/621-1262
The Flag Shop in Union Station has them also.
According to Chapter 1.20 of the City of St. Louis Revised Code, the City of St. Louis flag is the following:
"The flag with a solid red background has two broad heraldic wavy bars, colored blue and white, extending from the left top and bottom corners toward left center where they join and continue as one to the center right edge. This symbolizes the confluence of the Missouri and Mississippi rivers. Over the point of confluence a round golden disk upon which is the fleur-de-lis of France (blue) calling attention to the French background of the early city and more particularly to St. Louis of France for whom the City is named. The golden disk represents the City and/or the Louisiana Purchase. (Heraldically, the disk is a "bezant" or Byzantine coin signifying, money or simply purchase.)
The flag’s colors recall those of Spain (red and yellow or gold), Bourbon France (white and gold), Napoleonic and Republican France (blue, white and red), and the United States of America (red, white, and blue)."