I recently ordered some samples from the Natural Curtain Company who allow you to order 20 samples for free. I chose a range of colours and materials and although they are made for curtains I think some could be used for upholstery:
I also ordered some samples from a company called Dedar through Architonic. One sample was made of 31% Polyamide, 30% Linen, 25% Acrylic and 14% Viscose and the other 65% Jute and 35% Cotton.
And I’ve still got plenty more samples on their way!