There are several excellent sources for prairie/savanna seed mixes in Missouri. None of these list a mix specifically for savanna habitats, but I'm sure they could all recommend an appropriate mix for your site (or even make up a custom mix). I recommend that you deal with these local sources if you can.

Missouri Wildflowers Nursery (Jefferson City MO):
http://www.mowildflowers.net/index.htm

Easy Living Wildflowers (Willow Springs MO):
http://www.easywildflowers.com/Seeds.htm

Hamilton Seeds (near Houston MO)
http://www.hamiltonseed.com/Hamilton_mixes.html

There are several sources outside Missouri that sell savanna' seed mixes. Even though these mixes are tailored for savanna habitats, the fact that the seed sources are not local make them less desirable (IMHO).

Prairie Moon (Minnesota):
https://www.prairiemoon.com/store/template/product_display.php?NID=47&SID=d4bb5fc6d632625fe526e68013a80011

Ion Exchange (Iowa):
http://www.ionxchange.com/seed_mixes.html