You have to do frequent yard/estate sale and thrift shop canvassing to find specific items like yarn and fabric. Although I rarely do yard sales, I've picked up a lot of really nice fabric at estate sales, for great prices.
Sometimes, however, people can just be unreasonable. I went to a moving sale and the woman had rolls and rolls of high-end fabric. She had been to the local fabric stores and priced them, then wanted retail less 20%. No wonder she had so much left the last hour of the last day of the sale. I was the only person there and she still wasn't willing to budge. She asked me why people want to pay so little and I tried to explain to her that people are only willing to "settle" on color, prints, fabric content and yardage they didn't get to pick if the price is right. For what she was charging, they'd rather go to the store and pick whatever they want instead of what's there. She still didn't get it. I walked away and left her with her fabric collection - which she was going to have to pay to move.