The best readily available paper I have found for making cartridges is TRIMACO Easy Mask brown general purpose masking paper 6 in. X 180 ft. which is about $3.00 at Home Depot or Lowes.
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I just go to Dollar Tree near me (it's a dollar store, others are Dollar General, Everything's a Dollar, etc) and buy a $1.00 paperback book. Then I tear out several pages and cut them in half to the starting shape shown above.
Apparently one of the horrors of an army travelling through one's town in Europe and North America in the time of muzzleloading arms, was the damage to book collections as armies harvested books to fashion into cartridges. SOME of the reenactors with whom I participate, go to the trouble of photocopying text from an 18th century publication, and then use that paper to cut into cartridge paper....
LD