Have you tried your local rock landscaping place...
I don't know about where you are at, by ours down
here have rock from all over the US... and if not, they
might be able to get them for you, as to not have to
pay alot of dough for shipping...
It occurs naturally in Ohio, if you have a mind to and you don't mind the drive you might be able to talk some property owners out there to let you gather some of it. A friend of mine who lives in Pemberville has given me several pieces of it.