If you prefer to save money with a do-it-yourself approach, MOBOT has an excellent step by step guide for siting, designing and constructing a rain garden.
Washtenaw County, MI Master Raingardener Certification Class Videos
The first three classes in this series include excellent guidance on site selection and preparation. We do not recommend many of the plant choices that appear mostly in class 4 and 5 as they include many cultivars and non-native plants, some of which are invasive in Missouri. Instead we recommend these plants that are native to the St. Louis's ecoregion.