You're so welcome! I haven't actually watched MLP so I had to google Fiddlesticks, but it looks like fun! All the best! If I can help with anything else, or if you need a pump-up pep-talk, I'm your man!
If you want to make the whole hat from scratch, maybe a strong white felt? If you're pressed for time and don't mind altering a bought piece, how about searching online costume shops for a white fancy dress stetson hat? You could alter it to cut out the front bit. I think if you google 'Lone Ranger' costume you might get some hits ???