Part of the ‘Antique Fabrics‘ series, this fabric isn’t necessarily extinct, but it was more popular in the past than the present.
Kersey is a coarse woolen cloth, originally made in Kersey, in England, hence the name. Made from lower-quality wool with a twill weave it was thick and sturdy, and very warm. Very suitable for peasant / working-man wear, if you can tolerate wearing wool. It was the mainstay of the medieval woolen industry and thus very common during that time – and for quite some time thereafter.
For a good-enough equivalent, there are a lot of wool twills out there and some Civil War suppliers offer a modern version – William Booth Drapers calls it jean cloth. It’s a matter of finding something with a sufficiently rough hand – not popular for garments today, admittedly – and an appropriate color. Consider something tweedy, or a woolen serge if it’s sufficiently rustic in appearance – coarse and workmanlike is what you want.
Treat it as you would any pure-wool fabric – hand wash in cold water, lay flat to dry.