what does the potion do? Depending, they might want to have an item or hair or eyelash or nail clippings, sweat or tears (that would be hard to get) from the bad guy himself?
The potion must grant the person who drinks it the strength to successfully banish the Bad Guy from the mortal world, since he doesn't belong there anymore. (Any more detail would be spoilery.) It might be kind of hard to get anything physical from him, though!
Hair from a one-eyed hound dog, a heart shape stone found in the depths of the Mississippi, the fingernail from a spinster school teacher, a firefly's heart
Ooo, I like the firefly's heart! Though squishing the bug might be gross.

I'm also imagining that there are some kind of local insects whose parts could be harvested (under the full/blue moon at night, of course!)....
What about local plant life?
Oh yeah, good idea! I remember seeing all sorts of bizarre creepy-crawlies in Missouri when I visited. Stick insects, cicadas, katydids, spiders carrying baby spiders on their backs...
As a kid raised in Missouri in the 50s - 60s, I feel like I ought to be able to come up with some authentic show-me state stuff. Is it rural Missouri? Wild pawpaws. A sycamore puffball. Sassafras root. A dogwood twig (dogwood became the state tree in 1955). Uh... how about a chunk of limestone from a quarry? Or a crinoid (the state's official fossil). Or a bluebird feather (state bird). Water from the confluence of the Mississippi and Missouri rivers.
If it's urban, maybe toasted ravioli (popular in St Louis from the late 40s to present). A piece of gooey butter cake (supposedly invented in the 1930s or early 40s by a St. Louis baker who who accidentally reversed the proportions of sugar and flour in his cake batter). Provel cheese. (I must be hungry; everything I think of is food.)
Whatever you decide, I'd love to read a book set in 1955 Missouri!
These are fantastic, Ruth! I may have to pick your brain more about 1955 Missouri. If you have any more reminiscences to share, I'd love to hear them.
A blind cicada 
A 13-year cicada certainly would be hard to find...
A gust of wind from a tornado.
I like this! Reminds me a bit of Savvy by Ingrid Law, where the grandma cans songs in jars.
Thanks for all the great ideas, guys! Keep them coming!
Karen