Yup - another idea from Pinterest...
Take any CD holder. I got this one at IKEA for $3.
And you can use the pockets for bulletin board letters. I put the letters in order on the left side, and numbers and punctuation marks on the right side.
On Pinterest, I saw a teacher use a big binder with the CD sleeves to use. I didn't need one that big, so this one was perfect. I'm sure the younger grade teachers would need a bigger one! :)