The disappearance of silkspan was news to me. I did some digging on the web; apparently it's creating a crisis in the flying model airplane world. K&S, which apparently supplied the stuff for most "stick and tissue" airplane kits, has quit selling it, and says it's "no longer being manufactured."
Bluejacket still lists it on its website. I just sent an e-mail to BJ, asking if it has a good stock of it left. If so, I want to stock up.
I've never found a material that quite matches silkspan for furled sails - at least for the tricks I use (painting it with a mixture of acrylic paint and white glue, letting it dry, then touching it with a water-dampened brush to soften it for the actual furling process). Ordinary tissue paper tends to disintegrate when wet under the rigors of furling. And there just isn't a fabric that's thin enough for furled sails on anything but the larger scales. (I did try some genuine silk, which is sold to RC airplane modelers, once, but I didn't like it much.)
I can suggest three possible sources of stuff that certainly looks and acts like silkspan - though I have no idea how long they'll be available.
1. Lens tissue, from a camera store. This stuff is extremely thin, and works well. (I used it for some of the sails on my model of the frigate Hancock.) The drawback, obviously, is that it only comes in quite small sheets. On the other hand, a package of lens tissue will last a long time - and doesn't cost much.
2. A few months back I bought a nice little HO-scale Skipjack kit from a now-defunct company called Maritime Art. The material supplied for the sails is, of all things, a coffee filter. It seems to be mighty similar to silkspan, though maybe just a tad stiffer. I haven't built the kit yet, but it looks to me like the coffee filter should work just fine.
3. Arts and crafts stores (like Michael's and A.C. Moore's) sell a big variety of papers by the sheet. One of them is "rice paper." It has a weight and texture about like silkspan's, though the fiber structure of it is a lot more visible. I haven't tried this one either, but it looks like it might work.
I suspect the airplane model firms will come up with a substitute. Till then, I've got enough lens tissue on hand to last me for a while. I also intend to order some Bluejacket, if the firm still has some.