Free, I can't help you with. However, I suggest garage and estate sales. Sometimes it's a crafters estate that's being sold off and you hit the jackpot. Pricing depends on who is running the sale, but I've seen a tub of yarn go $5.00 That's a pretty good price when you consider that it can be as much as $7.00 a skein at the craft stores.
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Often a manufacturing plant will have scraps left over of anything they are making- look online and see if there are any textile manufacturers you can contact. If you were in the Bay Area of Calif., there is a place called "RAFTS" that has lots of stuff like yarn- not free- but super cheap.