Large circuit board options
Try directly contacting a PCB fabricator.  They usually buy large pieces like this to allow making bulk multi-up boards.  They might give you a good price on some surplus they have.  They would normally buy in large quantities due to their high volume of  manufacturing.  Trying to buy small quantities yourself from the OEM would likely be very cost prohibitive.

In a few years you might be able to use a conductive ink printer to make a PCB, similar to using a 3D printer of today.  These are just starting to be available.