No Orange Homework Book

I accidentally sold my girlfriend's child's homework book (the grade school orange one). I used the bookshelf and it said I owned an orange and a blue, but they're absolutely nowhere to be found. I chose the child and went to buy the books as him, and the orange book was gone from all bookshelves in my house and the computers. He hasn't aged up at all. I can't figure out how to get this book back so he can do his homework. I spent over an hour researching and found nothing about the book just vanishing from being purchasable. Help!