You need to find the right paper stock, supplied by a company
pitched at the retail end of the paper-conversion market who'll
do smallish orders -- quite hard to locate, as it turns out.

Music-roll paper is a bit thinner than commercial writing paper,
about 60 gsm (grams per square meter, the normal European measure
for paper) instead of 70-80 gsm for writing, so you need to go to
an industrial supplier of "technical paper".

It's hard to make the breakthrough, and I'm very grateful to have
access to the supplier identified by Tom Jansen of Musikwerkstatt
Monschau.  He'd had great difficulty until someone said, "A player
piano!  We had one, I know exactly what you want," and turned up
the ideal paper.

The supplier is Blumberg Systempapier in Germany.  The stock is
a plain white paper of exactly the same thickness as original roll
paper, and unwaxed, just like original piano rolls.  Now they have
the specification in their system they actually supply this to me
labelled "Für Musikinstrument"!

  http://www.blumberg.de/en/customized.php 

When I first ordered the stock was what they used for medical
chart recorders, a very authentic off-white, but they now supply
a high-white of identical specification.  As Blumberg are retail-end
converters they'll do quite small orders to your exact specification.
I get a couple of dozen reels at a time, 500m long and 300mm width,
about 11 kg per reel.

As an aside, delving into paper specifications, industrial papers
as used for rolls come with all sorts of technical specifications.
Obvious is weight and thickness (paper of the same weight can be
of different thicknesses, depending on its degree of compression).
Tensile strength and degree of stretch are also obvious, and differ
along and across the sheet.

Less obvious are things such as burst index (resistance to having
holes punched in it), roughness (which matters if you don't want
rolls to slip during play), and tear index (resistance to fraying of
the edges).  Runability is also important, relating to whether the
paper weaves from side to side as the roll plays.  It's one thing
to know these exist, quite another to know what values are suitable!
You need the experience of suppliers such as Blumberg to pick a stock
suitable for the stated needs.

For interest, here's a specification sheet for "Swanwhite" paper,
which is a candidate stock for music rolls.  You can see the range
offered and its details:

  http://www.billerudkorsnas.se/Global/Our%20offer/NEW%20products/MF/Swanwhite.pdf 

And if you like huge machinery, paper-making offers some of the
very finest, making reels of 120 tons at 1200 meters per second --
a remarkable mix of fragile and massive.  There are plenty of YouTube
videos about paper-making to waste an evening on.

Julian Dyer