I bought a metre of 160cm wide white japara (140gm/sq m) from a Spotlight shop in Melbourne the other day for A$7.80. I'm going to use it on a set of sails for a 27" long model that was given to me nearly 65 years ago and which has been in storage for a lot of that time. The original sails (beautifully sewn) have deteriorated and can't be used, so I'm now about to learn to use a sewing machine! If anyone wants any of this japara, I'd be happy to buy what is wanted and post it.
I also recently obtained from SAILSetc Ltd in Witham UK an excellent book called "Ralph's Guide to Vintage Sailmaking - Cotton Sails for Older Styles of Model Yacht" by Ralph Nellins, which gives lots of excellent instructions in how to do it. Could be helpful to others.