I bought these rocks at Walmart for $1.47 in the aisle where the aquarium accessories are located. Quick side note they had the same rocks in the craft department but they were $5.00 a bag. Then I used paint markers to write Capital letters on one side and the matching Lowercase letters on the opposite side. After they dried I sealed them with clear fingernail polish (it's just what I had lol).
They were a huge hit, my boys love them!!