I'd love to buy from Frame Destination, but until they begin to sell blanks, it aint' gonna happen.
You can currently get any size blanks you need from the folks at www.Redimat.com

Their outside cuts are all straight cuts. With a few suppliers over the years, I have seen some selling their drops (the beveled cut from the inside of the window mat) as blanks and that's not the same thing if you have to worry about trimming the outside off before you can even cut a window in it. Even worse, I have seen so called mat cutters that are selling cheep window mats that are beveled on the outside as well.