The Only Way to get Fox Fur is by catching Foxes in the wild and breeding them in a Fox Pen. It will give you a permanent passive income of Fox Fur and other resources if you feed them.
Its a material only used to craft Boar Fodder. Essential to keep up the mood and food in the Boar Pen.
Check out this guide to learn more about Pens and how to tame and breed Foxes.