Thrift stores. I have found the best weight to be ladies blazers/jackets, but anything that says 100% wool and Dry Clean should work. I used to cut them up first, but recently have just thrown them in as is. Put them in a zippered pillowcase as some will generate a LOT of lint and the pillowcase keeps it out of your washer's lint system or your drain. The lapels and jacket fronts often have a fusible interfacing. Sometimes this will just pull off after the felting process, sometimes you can use these pieces with the interfacing left on, and sometimes you just need to discard or use for another purpose. I've been making pincushions with other found items from the thrift store (candle votives work great) left over pieces of wool batting, then a felted wool top.