A hornbook was a primer for children consisting of a sheet containing the letters of the alphabet, usually mounted on wood and protected by a thin sheet of transparent horn and often had a handle. The sheet, which was first of vellum and later of paper. Usually it had the alphabet in both upper and lower case, a listing of the vowels and then the consonants, Roman numerals and the Lord's Prayer. Sometimes there were other educational items on it such as phonetic word combinations or math facts to be practiced.
Your can make one yourself. A small wooden cutting board is in just the shape of a hornbook. Many have a groove around the edge, which serves as a frame.
Cut a piece of paper the size of your board and write what you wish to have on your hornbook on it. Glue onto the board and coat with Mod Podge to seal.
If you do not want to use a wooden cutting board, you could just cut a piece of cardboard in the shape of a hornbook and attach the paper to that.