For plot reasons, they need to have yarn. (No, seriously, my plot revolves around yarn. They don't necessarily have to have woven fabric, but dye-able cording is a must.)
Who's got a reasonable plausible idea for what fiber they are using? It can be either grown/husbanded within their small fortified town (goats?), scavenged from the forest (cedar bark?) or obtained in trade.
ETA: This is one of those fantasies set in an invented world with no "our Earth" charaters popping round to it. And it's not an alternate history. Since there's no history to be faithful to, picking a fiber is mostly a matter of getting the right feel. I am trying hard not to default to the European. And generally trying to write about people of color without being a jerk.
Thanks for all the help below. I'm starting to lean towards either goats or skin cording instead of yarns.
One of these days I'm going to write a novel that's vaguely Celtic and under-researched, just like everybody else.