Look on Ebay. Several seller sell Eastern Black walnut for very reasonable money. I have bought a dozen plus blanks from there. Most is not kiln dried, but some is so be careful. I do not like kiln dried for stock work. Mine was green and I seasoned it in the wood rack. What ever you get you should test the moisture content to be sure. Better yet buy multiple blanks and let them season for later use.
One seller had a pallet of maybe a hundred plus, two piece, plain Black Walnut blanks he was trying to sell. All air dried for years, decades and ready to go. Worth sending an Email to him and see if he would sell you ten or twenty for a buy it now price. I have bought several items that way when the entire lot was too big to deal with and I wanted a fraction of the total. This was the item.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250575837979This item has ended but I am sure that they would be happy to sell you some if that is what you are looking for.
http://stores.ebay.com/Wingo-Wood-and-Lumber-CompanyThis seller sells mostly fresh cut wood if you have the time to let it dry. I have bought several from him and the wood was better than the pictures showed. I by crotch Black Walnut from this seller, but he does have straight grain wood also.