I saw stones with letters on available to purchase from an educational catalogue, but thought I could make these very easily. These have been in use now for well over a year and still going strong no fading despite being stored outside. I simply purchased bags of medium sized stones from a garden centre. The advantage of making your own letter stones is that you can make stones with children's names on, significant words or blends.
I also made number stones starting with the smallest stone for 1 to biggest for 10. The only difficulty with these stones is they get muddled up with the other collections of stones and those in the garden.
"I never teach my pupils; I only provide the conditions in which they can learn." Albert Einstein