If you guys are looking for White Cedar you might want to check with lumber suppliers in Eastern Ontario and Quebec. Now I bet your saying "yah I'm going to import that from Canada". Well you may not have to. You could e-mail them and ask which American companies to they sell to. Don't know if it will help or not, but could be worth a shot.
I know of a place (about 1/2 hour form here) that deals primarily with cedar, not 100% sure that it's white cedar, but likely wood be. Here's there addy if anyone is interested.
http://www.woodviewproducts.com/lumber.htm
I don't know how large an operation these guys are, but there are some large mills in Ontario and Quebec that might solve your problems.