I'd try West Penn Hardwoods in Conover. He seems to have endless slabs of all kinds any time I've been there and he's close enough to go have a look in person.
Steve Wall Lumber in Mayodan. But it will not be cheap. (I bought some about 12 years ago and he had the best price back then at about $10/bf for 8/4 -- now $14/bf for 12/4). Note that the prices I used are not for slabs.
Cormark INtl. in Weaverville, NC (just north of Asheville) would certainly have what you need. Their web prescence is mainly turning blanks, but they have large amounts of everything in all sizes.