This is the most popular silk thread, it is readily available
in North America. The 'Gossamer' is the usual thread used.
Pearsalls
http://www.dtnicolson.dial.pipex.com/page57.html

This is another brand which came on the market in 2007.
It is marketed in Europe by Coats & Clark.
Seta Reale
http://www.dtnicolson.dial.pipex.com/page183.html
The (B) type is similar to Pearsall's Gossamer in thickness
but there is a huge range of colours.
The (A) type is much thicker, but still looks useful for Traditional Wet Fly bodies (Bergman).

Don't know much about Kevlar except for use on spun deerhair.

The Canadian Unithread is graded 6/0 (medium) and 8/0 (fine). An excellent thread.