I lived (in Columbia) and worked all over Missouri for 7 years. Missouri is my second favorite place in the U.S. except for Florida from Orlando north, which is my home. Had some great times there and much of the Ozarks is still wild and full of killer places to explore.
I saw copperheads (where I lived in the sticks was a wildlife area in Ashland loaded with them), cottonmouths (near the nothern range limit on Ft. Leonard Wood in Pulaski Co.), racers, rat snakes (once I caught one under a tree stump in Carter Co. munching on a rat of some sort), ribbons, garters by the buttload, water snakes, ringnecks quite a bit, rarely a Timber rattler (in Ozark Co.), and two speckled kings. Never saw a prairie king, milksnake, or Maussagua Rattler though.
The glades of Missouri are cool for some snakes, collared lizards (they are state protected though), and other lizards/skinks. I use to see my snakes in all types of habitats, but sinkhole ponds, deep ravines, and creek beds were always places to see snakes. But in the morning, especially in the spring, glades were perfect spots for snakes to sun and I have some really cool stories of the stuff I saw while buried 8 miles in the Ozarks on some glade watching herps eating other things!
The cave areas and springs also have endemic, cool as# salamanders too!
You have hellbenders too, in the really better streams in the Ozarks - I only saw one once (Texas Co. around Licking) and it was really cool!
Try Hickory Canyones Natural Area in St. Genivieve Co. (steep ravines with moist cliffs, waterfalls, strems, and nice upland forest), or Valley View Glade Natural Area in Jefferson Co., which is only a few minutes from St. Louis (I use to live by the Missouri Botanical Garden); it's great site but don't collect without permisson. Iron Mt. and the area around it in Reynolds Co. is also killer.
The s.western part of the state with glades/prairies, and the Ozark Ravines is where I saw most of my snakes.
You're lucky, MO is a great state with plenty of herps to find - you may have to hump it a bit to find the good stuff, but trust me, it's worth it man!
Peace.
-John