www.darhmatrading.com has a lot in the way of dye ready fabrics...which is to say it is free of any fillers and is just the fabric. They have many natural fiber fabrics. It is primarily a dyeing supply house but they have blanks in clothing, accessories and yardage. Prices are reasonable as is shipping. I buy from them to make my own hand dyes and have used the canvas. It seems to me that would be a good resource for chemical free canvas which is what you are looking for.