One of the material I love to use is noble sweaters made out of noble fiber. I recycle cashmere, angora, mohair and merino wool like there's no tomorrow. Here's a couple of tips if you want to start hunting for these treasures or if you already have a sweater ready to be transformed.
My trick is to always keep an eye open. I look mostly is thrifts shops and in second hand stores. With time I developed a magic touch, I usually know if a sweater is made out of natural fiber by rubbing it between my fingers. You just have to look at the label. If the composition suits you and the color is nice, then it's a no-brainer.
It all depends on what you want to do with your sweater. To make dolls, I sometime opt for sweaters with a mix synthetic and natural fibers if the knit is beautiful and doesn't fell too "plastic". If I want to make mittens, I will look for something 100% animal fiber. There's also different quality of the same fiber, for instance merino wool is not always as soft, feeling it will help you choose what to do with it. It's very rare to find a sweater that's 100% mohair or angora, they're so expensive and slippery that they're usually mixed with others. Then again you have to choose if the mix is acceptable to you, mainly by touching it and by thinking of what you want to use it for. Personally, if it's mainly mixed with natural fiber, it's coming home with me.
Stains and holes
Chances are a wool sweater found in a thrift shop will be well worn. Since they're quality fibers, they last very long but might have holes and stains. Mites prefer them to other fibers, so holes are almost inevitable, especially in cashmere. If the holes aren't too big or numerous, you can cut small pieces in the sweater, or even fix the holes with a bit of mending. As for stains, it's a bit of the same solutions. They probably won't come out, so better plan to cut around it. If the sweater is full of stains, better let it go unless you a pro in dying.
Most animal fibers don't retain odors. Once at home you can hang your sweater outside to let it air out a couple of days. You should wash it by hand or in the machine on delicate cycle. Always wash your knit with cold water, a gentle soap and let them air dry. If there's holes you should repair them before the wash so they don't expand. You could also make your sweater shrink by washing in cold and hot water, with lots of agitation and putting it in the dryer. A knit that's been shrunk won't unravel and can be use just like felt. Careful, it will become much smaller and thicker if you do this.
Recycled knit are sewn pretty much like any other knit. Some will unravel more then others, so you have to be more careful with the finishing. If you wish to keep its elasticity, you should sew with a serger or an elastic stitch. When I use them for doll making, I don't really mind all that. I usually sew with a straight stitch, and the less clean edges are part of the charm of the doll. You can see a couple of example in the photos below of what I like to do with wool sweaters. I love to use cashmere for doll hair or hats, I think it gives them a bit more of a "hair feel".
Anyway, hope this helps and that next time you find a extra soft sweater you'll start thinking of all you can doo with it. Happy sewing!