Turtle Soup in St. Louis Area
The place to get turtle soup is in the metro east, over in the Fairview Heights/O'Fallon, Illinois area. On Saturday mornings in the fall, fire stations cook turtle soup in a somewhat German fashion and sell it until the soup runs out. So, you bring your own tupperware and get it filled up while helping support your local area firefighters.
As for other times during the year, I haven't run across any sign of turtle soup on a menu in St. Louis. I served it once last year for a week when I was longing for the kind of turtle soup served at Commander's Palace, and I was surprised with how well it went over.