Rain Gardens: A Guide for Homeowners and Landscapers is a good book for information you need. It can help to place and design a garden. Once you have a plan in place, select the plants for your garden. Choose plants by height, bloom time, color - whatever your preference.
The DNR has a listing of Native Plant Nurseries.
Schools may be eligible for free native plants for rain garden projects at your school site. Email us to see if your school is eligible!