I used a wool blend felt that I purchased from this etsy store. There are many store on etsy that sell 100% wool like here and here. The blend I use doesn't pill or stretch and is a nice alternative to the 100% wool, which can be pricey. You do not want to use the polyester felt found at craft supply stores. It will pill and stretch and you won't be happy with your finished product.
I traced around my snap clip (I used Goody's) and then added about and 1/8" seam allowance.