Sorry, but no public transit currently to SFSL. Current financial woes have really reduced Bi State Service throughout the city, however after August 3 they are restoring the Eureka express service:
http://www.metrostlouis.org/MetroBus/Maps/RouteMaps/Map410X080309.pdf
http://www.metrostlouis.org/MetroBus/TimeTables/410X080309.pdf
Problem is, that it's based on rush hour ridership so you can get there in the evening, but not in the morning, or return in the evening, but not go out there at night. Plus it's quite a hike from the park and ride lot to the park.