Colin,
I am in Columbus Ohio. I use Sommers Wood & door, P&W Millworks, Frank Paxton Lumber and so many others. All 3 that I have listed sell and stock a lot of different species in 4/4, 8/4 as well as many different sizes. They can get anything for you in short lead times. They also have their own milling department so that can provide any thickness, wide and length. Plain it, joint it, sand too. They can do glue ups for you as well. These companys also specialize in other areas as well such as Sommers, they make raised panel doors and such. If you tell me where you live I can probly find someone closer to you. Ohio has a lot of good mills.
-Dave