That wouldn't be a local stock item at a
paper supply house AFAIK.
Label companies buy the large rolls and cut them down as needed, using machines that
are very similar to what vinyl cutters are.
I have a feeling that you will be hard pressed to find that width (or larger which could
be cut down to 51")
If a local
paper supply house is of no use for material or even a lead for some there may be
a label company in your area and they might help you find a supplier. If they won't
help thinking you might be a competitor you could always check their dumpster to see
if you can find a brand name or two...otherwise hit the Thomas Register.
You might wish to check out Flexcon. I think they started out in the label
paper business
before getting into vinyl and metallics.