I get mine at thrift shops, flea markets and yard sales. Be on the look out for old blankets, men's and women's firmly woven coats, jackets, trousers and skirts. Check for fiber content. I take apart, remove fasteners and linings and wash gently in hot water, damp dry and lay the pieces flat to finish drying. I have three bundles of ivory/natural from wool/cashmere coats that I am going to dye.
A lot of wool can be gotten from a ladies' coat purchased for 25 cents and it would probably been landfill otherwise.