Assalamualaykum . If someone buys books or movies which contains elements of shirk or kufr in the movie or the book itself or in the name of the movie or the book will he become a kafir or a mushrik because he is supporting the creators of this things by buying them ?
Becoming a kafir/disbeliever has solely to do with the intentions of that person. If you just watch it for entertainment then obviously it is not doing shirk. But instead, if you actually believe in anything that has shirk element in that, then that would surely be kufr.
Prophet Muhammad (SAWW) once said "The reward of deeds depends upon the intention and every person will get the reward according to what he has intended." (Sahih Bukhari, Kitab-ul-Imaan, Hadith No. 53)
This hadith itself clears many misconceptions that we mostly have.