For "historical" models, especially square-riggers, Dupont Supplex Nylon is becoming the preferred material, on this side of the pond at least. It's the material SC&H uses in their square rigger kits.
It's strong, low stretch, UV resistant, water-proof, melts (ie: can be cut with a hot knife and heat sealed against fray, holes made with a hot point that won't run, etc), comes in weights light enough to see through (ie: near scale weight), looks great just hanging there, and a bunch of other good things.
The source the fellas here have been using is:
Rockywoods with 36 colors of 6oz/yd at $5.79 USD/yd for 1 to 9 yards.
There may be other sources on the east side of the ocean.
Pictures:
RC Royal William w/Supplex SailsTop'sl, spanker, and jibs for 1:36 ConstellationSC&H brig finished as US brig-or-war SyrenJust can't see enough pics of this modelCloser lookThere's other pics you can get to from those. :)