i wanted to do the similar - take home some clay - from my trip thru wyoming a few years back. a beautiful state. except no where did i see a spot that wouldn't be disrupted by my removal of some local dirt. so i didn't take any home...
that state has so much clay color that i would love to move there!
as a last resort i bought a knife that had various colors of wood laminated togethere to aproximate the local clay strata to me.
put the state on your list if you've never been there!
see ya
steve