The Wesleyan Publishing House no longer carries the Classic Catechism. The Free Methodist Church has ceased selling its book through WPH and has opened up its own online bookstore.
So, you can purchase the Classic Catechism here, Light and Life Bookstore.
Currently, you cannot purchase the Classic Catechism at Amazon. I suspect that with the switch from WPH to LLB the supply was cut off. I will work to get it reinstated at Amazon.