"I'm in central IL, roughly halfway between Chicago and St. Louis"

What city? I grew up in Decatur/Warrensburg, although currently in Boston.
The Lowes/Menards search... You gotta look through all the stacks and seek out the perfect boards. If the stacks are full, you might find one or two (or more) boards. Central Illinois has some lumber yards that should carry cedar. Much more expensive than Lowe's/menards, but it should be straiter grain. Don't forget pine or white board. Of the three boats I've built, only one has WRC and then its only half WRC. They are heavier but easier to find.
Unfortunately you have to call around. Concidering every major city is an hour away in Central Illinois, it might be worth traveling if the phone call verifies strait grained lumber (or a large stack of cedar at lowes)
Good Luck,
scott